Document IDE import

Author: smarter

docs/docs/contributing/eclipse.md

@@ -6,7 +6,7 @@ title: Building Dotty with Eclipse
 Build setup
 -----------
 
-1. Run `sbt eclipse`
+1. Run `sbt ;managedSources;eclipse`
 
 2. It is recommended to change the default output folder (in `Properties > java
    build path > Source`) to `dotty/classes` instead of `dotty/bin` because

docs/docs/contributing/getting-started.md

@@ -26,6 +26,7 @@ Start by cloning the repository:
 ```bash
 $ git clone --recursive https://github.com/lampepfl/dotty.git
 $ cd dotty
+$ sbt managedSources ## Needed for IDE import to succeed
 ```
 
 Dotty provides a standard sbt build: compiling, running and starting a repl can