Since the CodeIgniter project has put much more emphasis on open source and having others contribute back to the framework they love, they've had questions about the best places to get started and the steps to contribute back. They've posted this Contribution Guide to help answer some of those questions.
CodeIgniter is a community driven project and accepts contributions of code and documentation from the community. These contributions are made in the form of Issues or Pull Requests on the EllisLab CodeIgniter repository on GitHub.
There's a few helpful hints on things like submission guidelines, the PHP style guide for the project, PHP version compatibility, which branch to submit requests against and a quick how-too guide on getting up and running with git/github if you're not familiar with it.