Oftentimes open source projects place a CONTRIBUTING file in the root directory. It explains how a participant should do things like format code, test fixes, and submit patches. Here is a fine example from puppet and another one from factory_girl_rails. From a maintainer's point of view, the document succinctly communicates how best to collaborate. And for a contributor, one quick check of this file verifies their submission follows the maintainer's guidelines.

https://github.com/blog/1184-contributing-guidelines

sample: https://raw.githubusercontent.com/contribute-md/contribute-md-template/master/contribute.md

puppet example: https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

Let's create this file for Piwik as well!

The information can be found here: http://developer.piwik.org/guides/contributing-to-piwik-core

Maybe we link to this file, or we copy paste a summary of it in the CONTRIBUTING?