Add docs about how to use CMake in MSYS2

This closes msys2#144.

mkdocs.yml

@@ -65,6 +65,7 @@ nav:
         - docs/terminals.md
         - docs/package-management.md
         - docs/filesystem-paths.md
+        - docs/cmake.md
     - Development:
         # For MSYS2 developers, maintaining, creating packages, porting packages,
         # infrastructure, mirrors etc and everything else.

web/docs/cmake.md

@@ -0,0 +1,50 @@
+---
+title: Using CMake in MSYS2
+summary: There are a few gotchas regarding how to use CMake in MSYS2.
+---
+# Using CMake in MSYS2
+
+## Installing
+
+When building projects for Windows with CMake (as opposed to
+building projects that are going to run in MSYS2 posix emulation runtime)
+make sure to install the MinGW version of CMake, i.e. installing
+e.g. `mingw-w64-x86_64-cmake`.
+
+You also want to install a tool for actually doing the build. The
+current recommended default is Ninja, which you can install from the
+`mingw-w64-x86_64-ninja` package.
+
+(Other alternatives are `make` or `mingw-w64-x86_64-make`, for building
+with GNU Make running either as a MSYS2 or MinGW process.)
+
+## Building
+
+When running the CMake configuration command, it's recommended to explicitly
+specify the desired build file generator with the `-G` option. MSYS2
+provided CMake defaults to Ninja (but this is not the default in upstream
+CMake, so it's safest to explicitly specify it).
+
+Thus, to configure and build a CMake based project, you can run the
+following commands:
+
+```shell
+$ cmake -G Ninja <path-to-source> -DCMAKE_BUILD_TYPE=Release
+$ cmake --build .
+```
+
+The relevant generator alternatives are:
+
+* `-G Ninja`
+* `-G "MSYS Makefiles"`
+* `-G "MinGW Makefiles"`
+
+If building by invoking `cmake --build`, the same command works for all
+generator choices. Alternatively, to build by directly invoking the
+build tool, you can call `ninja`, `make` or `mingw32-make` respectively
+for those three alternatives.
+
+## Examples
+
+For building projects with CMake in MSYS2 in Github Actions, see the
+[CMake Example](https://github.com/msys2/setup-msys2/blob/master/examples/cmake.yml).