docs: 📝 document the "next" branch

CONTRIBUTING.md

@@ -18,6 +18,12 @@
 To setup the test environment, copy over the `.env.example` file to `.env` and fill in the required values.
 To then run the tests, run `npm run test`.
 
+## The "next" branch
+
+We have a branch called `next` that is used for features that are behind feature flags. All unreleased features / features still in development will be in this branch. If you want to contribute to a feature that is still in this state, make sure to branch off of `next` and submit a pull request to `next`.
+
+Next should also be kept in sync with `main` as much as possible, so that we can easily cherry-pick features into `main` when we are ready to release them.
+
 ## Pull Requests
 
 Make sure when submitting a pull requests that your code passes the tests, and that you have written tests for any new features or bug fixes.