Mention roadmap on CONTRIBUTING.md
Pull requests and Github issues are welcome!
+Planned improvements and changes are listed on the [H3 Roadmap](https://github.com/uber/h3/wiki/Roadmap). Roadmap items are currently discussed in Github issues. Feel free to open a discussion about an existing roadmap item or proposing a new one.
+
## Pull requests
* Please include tests that show the bug is fixed or feature works as intended.
