Add an Rcu.md documentation page for folly RCU
Add an Rcu.md documentation page for folly RCU

Summary:
folly RCU is a complex but powerful synchronization library. In order to make
the library easier to consume and understand, this patch adds an Rcu.md file to
the folly documentation folder.

Reviewed By: paulmckrcu, Gownta

Differential Revision: D35593896

fbshipit-source-id: 4cc1d4671f1aa5ef45f3e031a0ba0898d287b169

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly