Add pointers to main documentation

CONTRIBUTING.md

@@ -1,4 +1,14 @@
-Thank you for contribouting to PredictionIO. Please read and sign the [Contributor Agreement](http://prediction.io/cla). We cannot merge your PR without the agreement.
+Thank you for contributing to PredictionIO. Please read and sign the
+[Contributor Agreement](http://prediction.io/cla). We cannot merge your PR
+without the agreement.
 
-If you have questions about these terms please contact us at [email protected].
+For code contribution, please follow guidelines at
+http://docs.prediction.io/community/contribute-code/.
 
+For documentation contribution, please follow guidelines at
+http://docs.prediction.io/community/contribute-documentation/. Notice that since
+the documentation is inside the main code tree, signing the CLA is required
+before we can merge your PR.
+
+If you have questions about these terms please contact us at
+[email protected].