BLENDER: Add docs explaining merge strategy

Author: brechtvl

BLENDER_README.md

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+# Blender Merges
+
+Currently the process for merging upstream changes is to rebase, and keep
+Blender modifications on top. This keeps a clear overview of the modifications
+that were made.
+
+When merging a major new release, cherry-pick all the Blender commits on
+top of it. A simple `git rebase` will not work because the release and main
+branches diverge.
+
+First do changes in `blender-merged-develop`, and deploy on uatest. Then apply
+the changes in `blender-merged` and deploy in production.