Add statement about roadmap and contributions (#97)

Author: srowen

CONTRIBUTING.md

@@ -1,7 +1,18 @@
-## Contributing to spark-sklearn
+# Contributing to spark-sklearn
+
+## License
 
 When you contribute code, you affirm that the contribution is your original work and that you 
 license the work to the project under the project's open source license. Whether or not you 
 state this explicitly, by submitting any copyrighted material via pull request, email, or 
 other means you agree to license the material under the project's open source license and 
 warrant that you have the legal authority to do so.
+
+## What Can I Contribute?
+
+Generally speaking, fixes and enhancements are community-driven. A pull request is much more
+likely to be considered than a feature request by itself. A clear bug report with reproduction 
+valuable; even better is a pull request fixing the bug. 
+
+The roadmap is therefore driven by contributions, and releases to get these back into the
+hands of users. There is otherwise no fixed roadmap or release schedule.