Welcome to rustls-ffi! We're excited to work with you. If you've got a big chunk of work that you'd like to do, please file an issue before starting on the code, so we can get on the same page. If you've just got a small tweak, it's fine to send a PR without filing an issue first.
After you've sent a PR, we ask that you don't rebase, squash, or force push your branch. This makes it easier to see changes as your PR evolves. Once we approve your PR, we will squash it into a single commit, with the summary line set to the summary of your PR, and the description set to the description of your PR. That way we maintain a linear git history, where each commit corresponds to a fully reviewed PR that passed tests.
Check out README.md if you haven't already to find the conventions we follow. All code must be rustfmt'ed, which we enforce in CI.