link to both 2.11 & 2.12 nightly API doc, not just 2.11

Author: SethTisue

documentation/api.md

@@ -27,8 +27,12 @@ title: Scala API Docs
 
 ## Nightly builds
 
-* [Library API](http://www.scala-lang.org/files/archive/nightly/2.11.x/api/2.11.x/)
-* [Compiler API](http://www.scala-lang.org/files/archive/nightly/2.11.x/api/2.11.x/scala-compiler/)
+* Scala 2.11.x
+  * [Library API](http://www.scala-lang.org/files/archive/nightly/2.11.x/api/2.11.x/)
+  * [Compiler API](http://www.scala-lang.org/files/archive/nightly/2.11.x/api/2.11.x/scala-compiler/)
+* Scala 2.12.x
+  * [Library API](http://www.scala-lang.org/files/archive/nightly/2.12.x/api/2.12.x/)
+  * [Compiler API](http://www.scala-lang.org/files/archive/nightly/2.12.x/api/2.12.x/scala-compiler/)
 
 ## Previous releases
 * Scala 2.11.6