doc: add information about how to contribute

Inspired by the document at jitsi/jitsi.

CONTRIBUTING.md

+# How to contribute
+We would love to have your help. Before you start working however, please read
+and follow this short guide.
+
+# Reporting Issues
+Before you open an issue on GitHub, please discuss it on one of our
+[mailing lists](https://jitsi.org/Development/MailingLists) and wait for
+confirmation from one of the committers. Once you have that confirmation,
+please proceed to reporting the issue on GitHub, while providing as much
+information as possible. Mention the version of Jitsi Meet, Jicofo and JVB
+you are using, and explain (as detailed as you can) how the problem can
+be reproduced.
+
+# Code contributions
+Found a bug and know how to fix it? Great! Please read on.
+
+## Contributor License Agreement
+While the Jitsi projects are released under the
+[Apache License 2.0](https://github.com/jitsi/jitsi-meet/blob/master/LICENSE), the copyright
+holder and principal creator is [Atlassian](https://www.atlassian.com/). To
+ensure that we can continue making these projects available under an Open Source license,
+we need you to sign our Apache-based contributor
+license agreement as either a [corporation](https://jitsi.org/ccla) or an
+[individual](https://jitsi.org/icla). If you cannot accept the terms laid out
+in the agreement, unfortunately, we cannot accept your contribution.
+
+## Creating Pull Requests
+- Make sure your code passes the linter rules beforehand. The linter is exeuted
+  automatically when committing code.
+- Perform **one** logical change per pull request.
+- Maintain a clean list of commits, squash them if necessary.
+- Rebase your topic branch on top of the master branch before creating the pull
+ request.

README.md

 npm install
 ```
 
+## Contributing
+
+If you are looking to contribute to Jitsi Meet, first of all, thank you! Please
+see our [guidelines for contributing](CONTRIBUTING.md).
+
 ## Embedding in external applications
 
 Jitsi Meet provides a very flexible way of embedding it in external applications by using the [Jitsi Meet API](doc/api.md).