The PBR movesets on TwitchPlaysPokemon are now public and accepting community contributions!
Knowledge of YAML syntax, as well as Git and GitHub knowledge, helps, but is not at all required.
You can look at just about any set on this repo to see how to format a YAML Pokemon set. It's also briefly described in README.md.
To propose a change, go to the file you want to change and click the pencil to edit it. Make your changes, add a commit message and click "Propose File Change". This will create a fork for you; a copy of the repository that you can edit. You will then be prompted to make a pull request, a request for us to add your changes. Give it a meaningful title and body.
When you make a pull request, pokecat will automatically run and check for errors. Click the "Details" link next to the "compile" action if it says there are errors to see what you need to fix. If you can't figure out how to fix them yourself, you should turn "Allow edits from maintainers" on so we can fix them. To make more changes in the same pull request, go to your fork (it will be at github.com/YourUsername/pbr-movesets), switch to the branch that GitHub created (it will be something like patch-1), then navigate to files and edit them as before.
Of course, if you know how Git works, you can use that.
Our Issues page contains specific major goals we would like to reach. Look here if you want to know what you can help with. If you can't make a pull request to fix something small, don't use Issues; if you want to bring it to our attention, message a moveset team member such as Mathy.