Create main_branch.md

Author: foolip

rfcs/main_branch.md

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+# RFC #80: Rename `master` branch to `main` in all repositories
+
+## Summary
+
+Rename `master` branch to `main` in all repositories in the [web-platform-tests org](https://github.com/web-platform-tests).
+
+## Details
+
+TODO: which external repos hardcode the name of the default branch in any WPT repo?
+
+TODO: which repos need to maintain a `master` branch during a transition period?
+
+TODO: impact on open PRs?
+
+## Risk
+
+Calamity