update Windows build docs

   - mention 2017
   - describe 32-bit builds a bit more

Change-Id: I386c51bcf4863f91db2eade01bad1e54c1049e6c
Reviewed-on: https://skia-review.googlesource.com/15314
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed