Link to docs.rs in Readme

Author: phil-opp

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://travis-ci.org/phil-opp/linked-list-allocator.svg?branch=master)](https://travis-ci.org/phil-opp/linked-list-allocator)
 
-[Documentation](http://phil-opp.github.io/linked-list-allocator/linked_list_allocator/index.html)
+[Documentation](https://docs.rs/linked_list_allocator)
 
 ## Usage