If you wish to contribute please see any issue on GitHub tagged with "help wanted". Before starting to work, consider commenting on the issue you want to resolve or open a new issue to discuss design decisions and implementation details. This is so that no development cycles go to waste on implementing a feature that might not get merged either because of implementation details or other reasons.
Make sure to submit your PR's against the master branch.
See the README.md file on how to setup the project on your development machine.