Add Wangle README.md

Summary:
Facebook:
Moving from Dex.
Transliteration from whatever wiki format Dex uses to Markdown
Some minor corrections/additions.

Test Plan: Pasted into a private gist and it looks reasonable on github.

Reviewed By: davejwatson@fb.com

FB internal diff: D1199510

folly/wangle/README

-Please see https://our.intern.facebook.com/intern/dex/wangle/