diff --git a/NEWS.md b/NEWS.md
index ac73d39..60d4f35 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -24,6 +24,8 @@
 
 ### Notes
 
+- Document `?kit` with a package page that explains OpenMP threads, platform support, encoding behavior, and the main functions. Add "OpenMP and threads" and "Platforms" sections to the README, and add `library(kit)` to the first README example (#90).
+
 - Reduce peak memory in `funique`, `fduplicated`, `uniqLen`, `psort`, and `charToFact` by starting duplicate hash tables at 256 slots and growing them on demand (4x, capped at the previous full-size table) instead of allocating the full table up front. Rehashing re-reads the source values, so results are unchanged; inputs with few unique values use much less memory, while peak memory can be higher at growth boundaries for high-cardinality inputs (#74).
 
 - Speed up `vswitch` for string and list outputs by scanning `x` once instead of once per value and fusing default/`na` writes into the match pass; single-value lookups also avoid a separate initialization pass. Atomic outputs keep the value-major vectorized pass, with the first value fused into initialization. Last-match-wins behavior for duplicate `values` is preserved, mixed output item types now fail with a clear error, and list-output scratch arrays are heap allocated so large outputs no longer exhaust the C stack (#74).
diff --git a/README.md b/README.md
index 870b5fe..29ccda7 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,66 @@ install.packages("kit")
 install.packages("kit", repos = "https://fastverse.r-universe.dev")
 ```
 
+## OpenMP and threads
+
+`kit` uses OpenMP in `iif()`, `nswitch()`, `vswitch()`, `psort()`, and
+`charToFact()`. The other functions use one thread. OpenMP is optional.
+Without OpenMP, `kit` still works, but these five functions use one thread.
+
+- **Linux:** Install the OpenMP runtime for your compiler. With GCC, this is
+  usually `libgomp` (`libgomp1` on Debian and Ubuntu). With Clang, install
+  `libomp` (`libomp-dev` on Debian and Ubuntu).
+- **macOS:** Apple Clang does not include OpenMP. Install it with Homebrew:
+  `brew install libomp`. On Apple Silicon, Homebrew installs to a path that
+  clang does not search by default. Add that path to your R Makevars before
+  you install `kit` from source (`configure` decides with `CFLAGS`, so the
+  include path must be there):
+
+  ```sh
+  # ~/.R/Makevars
+  CFLAGS += -I$(shell brew --prefix libomp)/include -L$(shell brew --prefix libomp)/lib
+  LDFLAGS += -L$(shell brew --prefix libomp)/lib
+  ```
+
+- **Windows:** OpenMP is included in Rtools. Install the Rtools version that
+  matches your R version. Then install `kit` from source.
+
+Set the number of threads once for each session:
+
+```r
+options(kit.nThread = 4L)
+```
+
+The default is `1`. If you set a value that is larger than the number of
+available processors, `kit` uses the number of available processors. `kit`
+reads the option each time you call a function, so you can change it between
+calls.
+
+To check whether OpenMP is enabled, run:
+
+```r
+.Call(kit:::CompEnabledR)
+```
+
+If this call returns `FALSE`, the five functions above use one thread, even
+when `kit.nThread` is larger than `1`.
+
+## Platforms
+
+`kit` is tested on Linux, macOS, and Windows. OpenMP is optional. See
+[OpenMP and threads](#openmp-and-threads) for the installation requirements.
+
+`shareData()` behaves differently on POSIX systems and Windows:
+
+- **Linux, macOS, and other POSIX systems:** `getData()` reads the object one
+  time. A second call with the same name fails. If the sharing session stops
+  before `clearData()`, the object can stay in shared memory: on Linux in
+  `/dev/shm` until you remove it or restart the machine, on macOS until the
+  machine restarts.
+- **Windows:** `getData()` can read the object more than one time while the
+  sharing session is alive. If the sharing session stops, Windows removes the
+  mapping.
+
 ## Features
 
 ### Row-wise Statistical Functions
@@ -33,9 +93,10 @@ Vector-valued functions that combine several vectors or data frame columns eleme
 - **`pcount`, `pcountNA`**: Count occurrences of values or NAs
 - **`pfirst`, `plast`**: First/last non-missing values
 
-The word "parallel" in their names means they work across several vectors at once, not that they use multiple threads. These functions are single-threaded C code. Only `iif()`, `nswitch()`, `vswitch()`, `psort()`, and `charToFact()` can use OpenMP, and only when you raise `options(kit.nThread)` above its default of 1.
+The word "parallel" in their names means they work across several vectors at once, not that they use multiple threads. These functions are single-threaded C code.
 
 ```r
+library(kit)
 x <- c(1, 3, NA, 5)
 y <- c(2, NA, 4, 1)
 psum(x, y, na.rm = TRUE)  # [1] 3 3 4 6
diff --git a/_pkgdown.yml b/_pkgdown.yml
index 8991669..0008078 100644
--- a/_pkgdown.yml
+++ b/_pkgdown.yml
@@ -44,6 +44,10 @@ navbar:
 
 
 reference:
+- title: "Overview"
+  desc: "Package overview, threading, platform support and encodings."
+- contents:
+  - kit-package
 - title: "Row-wise Statistical Functions"
   desc: "Vector-valued (statistical) functions that combine several vectors passed as arguments, or a single list of vectors/data frame, element by element."
 - contents:
diff --git a/man/kit-package.Rd b/man/kit-package.Rd
new file mode 100644
index 0000000..cdf86c3
--- /dev/null
+++ b/man/kit-package.Rd
@@ -0,0 +1,39 @@
+\name{kit-package}
+\alias{kit-package}
+\alias{kit}
+\docType{package}
+\title{kit: Fast Data Manipulation Functions in C}
+\description{
+  \pkg{kit} provides fast, vectorised functions for data manipulation.
+  The compiled code is written in C. The package has no runtime dependencies.
+}
+\details{
+  \strong{Threads.}
+  Five functions can use OpenMP: \code{\link{iif}}, \code{\link{nswitch}},
+  \code{\link{vswitch}}, \code{\link{psort}}, and \code{\link{charToFact}}.
+  The other functions use one thread.
+
+  Set the number of threads with \code{options(kit.nThread = 4L)}.
+  The default is \code{1}. If you set a value that is larger than the number
+  of available processors, \pkg{kit} uses the number of available processors.
+  If \pkg{kit} was built without OpenMP, all functions use one thread and
+  this option has no effect.
+
+  \strong{Platforms.}
+  \pkg{kit} is tested on Linux, macOS, and Windows. OpenMP is optional.
+  On macOS, OpenMP requires \code{libomp} from Homebrew. On Windows, OpenMP
+  comes with Rtools. See the README for installation details.
+
+  \strong{Encodings.}
+  \code{\link{psort}} sorts text in the C locale by default. Set
+  \code{c.locale = FALSE} to use the session locale.
+  \code{\link{charToFact}} uses the session locale. Factor levels and sort
+  order can thus differ between operating systems for non-ASCII text.
+  \code{\link{vswitch}} and \code{\link{nswitch}} convert mixed encodings to
+  UTF-8 by default.
+}
+\seealso{
+  \code{\link{psum}}, \code{\link{funique}}, \code{\link{topn}},
+  \code{\link{shareData}}
+}
+\keyword{package}