Create readme.html using `configure_file`:
- Now it's modified only if final output changes (handled by CMake).
- If input file (from git) or blender version changes, it //will// be modified.
Since the output file stays unmodified for most developer runs,
install step installed it redundantly.
Also don't re-implement what CMake can do.
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Will need to test the `remove_recurse` part, so will not land it now. I had posted this diff to use phab as git stash but here we are (: