Since the output file stays unmodified for most developer builds,
install step installed it redundantly.
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.
Also don't re-implement what CMake can do.
Reviewed By: campbellbarton, LazyDodo
Differential Revision: https://developer.blender.org/D13863
14d98d0388