Add "contribute" section to README.md.

Add "contribute" section to README.md and link to the new guide on how to contribute to KairosDB documentation.

README.md

@@ -18,5 +18,17 @@ Installation instructions are found [here](http://kairosdb.github.io/docs/build/
 
 Join the [KairosDB discussion group](https://groups.google.com/forum/#!forum/kairosdb-group).
 
+## Contributing to KairosDB
+
+Contributions to KairosDB are **very welcome**. KairosDB is mainly developed in Java, but there's a lot of tasks for non-Java programmers too, so don't feel shy and join us!
+
+What you can do for KairosDB:
+
+- [KairosDB Core](https://github.com/kairosdb/kairosdb): join the development of core features of KairosDB.
+- [Website](https://github.com/kairosdb/kairosdb.github.io): improve the KairosDB website.
+- [Documentation](https://github.com/kairosdb/kairosdb/wiki/Contribute:-Documentation): improve our documentation, it's a very important task.
+
+If you have any questions about how to contribute to KairosDB, [join our discussion group](https://groups.google.com/forum/#!forum/kairosdb-group) and tell us your issue.
+
 ## License
 The license is the [Apache License 2.0](http://www.apache.org/licenses/LICENSE-2.0)