(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

README.md

@@ -1,6 +1,8 @@
 # django-serializable-model
 
 [![PyPI version](https://img.shields.io/pypi/v/django-serializable-model.svg)](https://pypi.org/project/django-serializable-model/)
+[![releases](https://img.shields.io/github/tag-pre/agilgur5/django-serializable-model.svg)](https://github.com/agilgur5/django-serializable-model/releases)
+[![commits](https://img.shields.io/github/commits-since/agilgur5/django-serializable-model/latest.svg)](https://github.com/agilgur5/django-serializable-model/commits/master)
 
 Django classes to make your models, managers, and querysets serializable, with built-in support for related objects in ~100 LoC (shorter than this README!)