Update main_branch.md

Author: foolip

rfcs/main_branch.md

@@ -6,6 +6,17 @@ Rename `master` branch to `main` in all repositories in the [web-platform-tests
 
 ## Details
 
+These are our existing repos, excluding archived and private ones:
+- https://github.com/web-platform-tests/data-migration
+- https://github.com/web-platform-tests/editor
+- https://github.com/web-platform-tests/rfcs
+- https://github.com/web-platform-tests/wpt
+- https://github.com/web-platform-tests/wpt-empty
+- https://github.com/web-platform-tests/wpt-metadata
+- https://github.com/web-platform-tests/wpt-pr-bot
+- https://github.com/web-platform-tests/wpt.fyi
+- https://github.com/web-platform-tests/wpt.live
+
 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?