Link to sprockets repository from README for convenience

Let's link directly to the sprockets repository where it is referenced
at the beginning of the README so that folks can easily follow the link
to learn more about sprockets.

Author: rmacklin

README.md

@@ -1,6 +1,6 @@
 # Sprockets Rails
 
-Provides Sprockets implementation for Rails 4.x (and beyond) Asset Pipeline.
+Provides [Sprockets](https://github.com/rails/sprockets) implementation for Rails 4.x (and beyond) Asset Pipeline.
 
 
 ## Installation