Clarify in the docs what belongs in portability/

Summary: This has always been the design rule for what belongs in `folly/portability/`, but was never written down anywhere except in my head, so put it somewhere people can actually find it so I can free that space in my head for other things.

Reviewed By: yfeldblum

Differential Revision: D5140587

fbshipit-source-id: 1f09c17b7ccfff780e1825670a7ffaa75cd1b1d5

folly/portability/README.md

@@ -10,3 +10,12 @@ 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.
+
+Note that before adding a new file to this directory you should determine
+whether the API you are adding is a portability header or just a platform
+dependent implementation detail. Only portability headers belong in this
+directory. A portability header is defined as a header that provides the exact
+API of some platform or configuration that is not available on all platforms.
+If the API being added does not already exist on at least one of the platforms
+Folly supports, then it is an implementation detail, and does not belong in
+this directory.