README.md: link to the list of supported platforms

It answers a question that someone might have when faced with the source
tree for the first time, and improves discoverability of the platforms
page.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37179

(cherry picked from commit c34ffd6c8b2859ef46873e6c3d6b3655da6db8c0)