UNREPL.el is a very young project, so contribution guidelines are fairly simple, straight forward, and very welcome.
Please refer to the issue tracker.
Feel free to open pull requests.
Be careful to create atomic commits with descriptive messages; when in doubt, please refer to these seven rules.
Make sure to test your changes before creating your pull requests. Tests run under buttercup, and you can get everything setup easily with Cask. After you have installed Cask, just run from the root of the project:
$ cask exec buttercup -L . tests