Patches are welcome in whatever form. Contributions will be distributed with Vimcat under a public domain licence. Providing a change to be included implies that you agree with this and your contribution does not cause trouble with trademarks or patents. There is no CLA to sign.
Issues are tracked on Github, but it is not a requirement to report issues there. Emailing the author is also fine. Feel free to report even the smallest problem, also typos in the documentation.
If the above text sounds familiar, it is modelled after the excellent Vim contribution guidelines. Basically, be a good open source citizen.