R1:daabc41fa108

Add detailed instructions for installing PHP on Windows

Summary: This isn't very obvious, provide some more specific instructions.

Test Plan: Followed the instructions on my Windows machine, got a working `php`.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2555