Clarify in the docs what belongs in portability/
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

rklabs/folly,rklabs/folly,facebook/folly,Orvid/folly,facebook/folly,Orvid/folly,Orvid/folly,Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,facebook/folly,facebook/folly,rklabs/folly