docs: Move README.win32.md to the docs folder

It’s a bit confusing to have two files called `README`, especially given
that `README.win32.md` only contains build instructions and none of the
other overview/contribution/bug reporting information that `README.md`
contains.

It feels like these would be better organised by having one top-level
`README.md`, and pointing to the Windows build instructions from there.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

README.md

@@ -13,7 +13,9 @@ The official web site is:
 
 ## Installation
 
-See the file '[INSTALL.md](INSTALL.md)'
+See the file ‘[INSTALL.md](INSTALL.md)’. There is
+[separate and more in-depth documentation](./docs/win32-build.md) for building
+GLib on Windows.
 
 ## Supported versions