Add guidelines to merging branches

docs/before-merge.md

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+# Before merging to main
+
+⚠️ **Hold up!** Before you merge that pull request, make sure to follow this checklist!
+
+## Any branch to `develop`
+
+If you wish to merge your branch to the `develop` branch, make sure to follow this checklist:
+
+- [ ] Run `elm-format` to ensure the correct formatting of the Elm files.
+- [ ] Use `elm-doc-preview` to verify whether the documentation is up to standards.
+
+## The `develop` branch to `main`
+
+The `develop` branch is the only branch that's allowed to merge to `main`. Once the branch merges to `main`, that indicates a new release on the Elm registry.
+
+Before that is being done, however, the following tasks should be done:
+
+- [ ] Run `elm-format` to ensure the correct formatting of the Elm files.
+- [ ] Use `elm-doc-preview` to verify whether the documentation is up to standards.
+- [ ] Remove exposed modules from `elm.json` that do not need to be exposed modules in the release.
+- [ ] Run `elm bump` to update the library's version number
+- [ ] Update the version name in the [default values config file](../src/Internal/Config/Default.elm).
+
+## Any branch to any other branch
+
+There are no limitations to merging other branches towards one another, although it is important to keep in mind that:
+
+- Contributors are advised to merge the `develop` branch into their branches regularly to avoid any merge conflicts.
+- Merging with branches that haven't been accepted (yet) might result in your branch ending up with code that will not be accepted.