Document how to make a portable build on Windows
On MSYS2, ./gradlew does not work as expected, so use its absolute path.
#
# "make release-portable" builds a zip containing the client and the server.
#
+# On Windows with MSYS2/mingw64, execute:
+# GRADLE="$PWD/gradlew" mingw32-make release-portable
+#
# This is a simple Makefile because Meson is not flexible enough to execute some
# arbitrary commands.
