ARROW-35: Add a short call-to-action in the top level README.md

Author: Wes McKinney <[email protected]>

Closes apache#13 from wesm/ARROW-35 and squashes the following commits:

e10bfc3 [Wes McKinney] Add a proper mailto link
c4428fe [Wes McKinney] Add a short 'how to get involved' blurb in top-level README

README.md

@@ -22,3 +22,24 @@ Initial implementations include:
  - [Arrow Structures and APIs in Java](https://github.com/apache/arrow/tree/master/java)
 
 Arrow is an [Apache Software Foundation](www.apache.org) project. More info can be found at [arrow.apache.org](http://arrow.apache.org).
+
+#### Getting involved
+
+Right now the primary audience for Apache Arrow are the designers and
+developers of data systems; most people will use Apache Arrow indirectly
+through systems that use it for internal data handling and interoperating with
+other Arrow-enabled systems.
+
+Even if you do not plan to contribute to Apache Arrow itself or Arrow
+integrations in other projects, we'd be happy to have you involved:
+
+- Join the mailing list: send an email to
+  [[email protected]][1]. Share your ideas and use cases for the
+  project.
+- [Follow our activity on JIRA][3]
+- [Learn the format][2]
+- Contribute code to one of the reference implementations
+
+[1]: mailto:[email protected]
+[2]: https://github.com/apache/arrow/tree/master/format
+[3]: https://issues.apache.org/jira/browse/ARROW