Add source path details to the CONTRIBUTING docs. (flutter#129)

* Add source path details.

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -17,6 +17,7 @@ file.
 * In the project structure dialog, configure an IntelliJ platform SDK
   - point it to the `Contents` directory in your just downloaded copy of IntelliJ Community Edition (e.g, `IntelliJ IDEA CE.app/Contents`)
   - name it `IntelliJ IDEA Community Edition`
+* In order to have the platform sources handy, clone the IntelliJ IDEA Community Edition repo (`git clone https://github.com/JetBrains/intellij-community`) and add a path to your local clone in the `Sourcepaths` tab of your `IntelliJ IDEA Community Edition` SDK and accept all the root folders found by the IDE after scanning.
 * Open flutter-intellij project in IntelliJ. Build it using `Build` > `Make Project`
 * Try running the plugin; there is an existing launch config for "Flutter IntelliJ".
 * If the Flutter Plugin doesn't load, check to see if the Dart Plugin is installed in your runtime workbench; if it's not, install it (`Preferences > Plugins`) and re-launch.