Suggestion: Provide Deprecation Warning for v13 in README.md

Hi, it took me some 2h today, until I found in https://www.patreon.com/posts/moulinette-for-128688508?l=de that this module is deprectated for v13 of FoundryVTT. My recommendation is, to make a short notice in the beginning of the README.md, so others will find this faster than me.

Cheers and keep up the good work!