README: Fixed typo, formatted code, and linked to docs

Summary:
(from @njormrod) This changes the READMEs to be .md,
appropriate for github. I viewed the docs on github, and they looked
WAY better as .md files.
Signed-off-by: Nicholas Ormrod <njormrod@fb.com>

Test Plan: view on github

Reviewed By: markisaa@fb.com

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

FB internal diff: D1999531

Tasks: 6783581

Signature: t1:1999531:1429220432:68c6d8dc70806d993c83cacce6369cb7e6e964c8

folly/wangle/rx/README → folly/wangle/rx/README.md

@@ -4,12 +4,12 @@ RxJava for JVM languages.
 
 It is basically the plural of Futures (a la Wangle).
 
-
+```
                     singular              |            plural
         +---------------------------------+-----------------------------------
   sync  |  Foo getData()                  |  std::vector<Foo> getData()
   async |  wangle::Future<Foo> getData()  |  wangle::Observable<Foo> getData()
-
+```
 
 For more on Rx, I recommend these resources: