StringCatalogEnum: added README

[superarts]

I purposely didn't leave a README for this utility, but based on the recent PRs, I feel like new developers may need a bit clarification. I'll provide a README soon.

[superarts]

This issue is to remind contributors to update https://github.com/superarts/XStringCatalog/blob/master/StringCatalogEnum/README.md and https://github.com/superarts/XStringCatalog/blob/master/StringCatalogEnum/USAGE.md in their PRs (don't push README to master directly like me)