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.
Will need to test the remove_recurse part, so will not land that piece of code now. I had posted this diff to use phab as git stash but here we are (: