merge dex and readme

Summary:
This is still manual as in the previous DEX.md revision, but I incorporated README.md into dex and redid the manual export.

Test Plan: reading_intensifies

Reviewed By: yfeldblum@fb.com

Subscribers: exa, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2124555

Tasks: 6047407

Signature: t1:2124555:1433370669:91f8bb55c1e6de4dbeccbb85556f54ae49464c7a