Readme: link both generated doc sites

Author: dhardy

README.md

@@ -10,7 +10,9 @@ See also:
 [![Build Status](https://travis-ci.org/rust-lang-nursery/rand.svg?branch=master)](https://travis-ci.org/rust-lang-nursery/rand)
 [![Build status](https://ci.appveyor.com/api/projects/status/rm5c9o33k3jhchbw?svg=true)](https://ci.appveyor.com/project/alexcrichton/rand)
 
-[Documentation](https://docs.rs/rand)
+Documentation:
+[master branch](https://rust-lang-nursery.github.io/rand/rand/index.html),
+[by release](https://docs.rs/rand)
 
 ## Usage