Windows build instructions documentation. (#6956)

Windows build instructions.

doc/windows.md

@@ -0,0 +1,42 @@
+# Windows
+
+## Building Ruby
+
+The easiest build environment is just a standard [RubyInstaller-Devkit installation](https://rubyinstaller.org/) and [git-for-windows](https://gitforwindows.org/). You might like to use [VSCode](https://code.visualstudio.com/) as an editor.
+
+Ruby core development can be done either in Windows `cmd` like:
+
+```
+ridk enable ucrt64
+
+pacman -S --needed bison %MINGW_PACKAGE_PREFIX%-openssl %MINGW_PACKAGE_PREFIX%-libyaml %MINGW_PACKAGE_PREFIX%-readline
+
+cd c:\
+mkdir work
+cd work
+git clone https://github.com/ruby/ruby
+
+cd c:\work\ruby
+sh autogen.sh
+sh configure  -C --disable-install-doc
+make
+```
+
+or in MSYS2 `bash` like:
+
+```
+ridk enable ucrt64
+bash
+
+pacman -S --needed bison $MINGW_PACKAGE_PREFIX-openssl $MINGW_PACKAGE_PREFIX-libyaml $MINGW_PACKAGE_PREFIX-readline
+
+cd /c/
+mkdir work
+cd work
+git clone https://github.com/ruby/ruby
+cd ruby
+
+./autogen.sh
+./configure -C --disable-install-doc
+make
+```