(docs): add releases and commits since badges

- same ones I use in other projects of mine - good to link to GH releases page for those that might want it and bc I usually use it as a changelog

Author: agilgur5

README.md

@@ -1,6 +1,8 @@
 # django-api-decorators
 
 [![PyPI version](https://img.shields.io/pypi/v/django-api-decorators.svg)](https://pypi.org/project/django-api-decorators/)
+[![releases](https://img.shields.io/github/tag-pre/agilgur5/django-api-decorators.svg)](https://github.com/agilgur5/django-api-decorators/releases)
+[![commits](https://img.shields.io/github/commits-since/agilgur5/django-api-decorators/latest.svg)](https://github.com/agilgur5/django-api-decorators/commits/master)
 
 Tiny decorator functions to make it easier to build an API using Django in ~100 LoC (shorter than this README!)