Added cutdown Futures.md into docs tree and removed futures/README.md.

Summary:
Futures documentation was drifting out of date. The documentation index also had broken links to Futures and a core readme.

This change adds basic futures documentation in the docs tree and removed the README.md in the futures tree.

Reviewed By: marshallcline

Differential Revision: D7937360

fbshipit-source-id: 120610cca21db5a8b38199a5098551f10bb85251

folly/docs/Overview.md

 ### `folly/`
 
-For a high level overview see the [README](../../README.md)
-
 ### Components
 
 Below is a list of (some) Folly components in alphabetical order, along with
@@ -92,7 +90,7 @@ Rabin fingerprinting.
 
 A polymorphic wrapper for callables similar to `std::function` but not copyable and therefore able to wrap non-copyable callables, such as lambdas that capture move-only types like `std::unique_ptr` or `folly::Promise`.
 
-### [`futures/`](../futures/README.md)
+### [`futures/`](Futures.md)
 
 Futures is a framework for expressing asynchronous code in C++ using the Promise/Future pattern.