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

Author: mhorne

README.md

@@ -12,6 +12,9 @@ Additional copyright information also exists for some sources in this tree - ple
 The Makefile in this directory supports a number of targets for building components (or all) of the FreeBSD source tree.
 See build(7), config(8), [FreeBSD handbook on building userland](https://docs.freebsd.org/en/books/handbook/cutting-edge/#makeworld), and [Handbook for kernels](https://docs.freebsd.org/en/books/handbook/kernelconfig/) for more information, including setting make(1) variables.
 
+For information on the CPU architectures and platforms supported by FreeBSD, see the [FreeBSD
+website's Platforms page](https://www.freebsd.org/platforms/).
+
 Source Roadmap:
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 | Directory | Description |