Added gitignores to the pregenerated metadata folder

These gitignore files should ignore the changes in the new metatadata if
a user runs generate.py.

In case the metadata does change and it needs to be updated, it can be
added with the -f switch.

This avoids unnecessarily big diffs when the user only regenerated 
metadata by either accident or to create a new set. 

I noticed this happened in recent pull requests so I thought it would 
be a nice thing to have.