A warning for the portability headers

Summary: [Folly] A warning for the `portability` headers.

Reviewed By: mzlee, Orvid, meyering

Differential Revision: D3014442

fb-gh-sync-id: f6557d8021dc2a50a656685492d717fdbff1124a
shipit-source-id: f6557d8021dc2a50a656685492d717fdbff1124a

folly/portability/README.md

+Warning
+=======
+
+These portability headers are **internal implementation details**.
+
+They are intended to ensure that Folly can build on a variety of platforms.
+
+They are not intended to help you build your programs on these platforms.
+
+They are, and will remain, undocumented. They are, and will remain, subject to
+rapid, immediate, and drastic changes - including full rewrites and merciless
+deletions - without notice.