Instruction to install django-simple-history before exectuing runtests.py is missing

[muneeb706]

Describe the bug
In the CONTRIBUTING.rst file under the Testing section, the instruction to install django-simple-history before running python runtests.py command is missing. Without this step, importlib.metadata.PackageNotFoundError: No package metadata was found for django-simple-history error is thrown.

To Reproduce
Steps to reproduce the behavior:

1. Run python runtests.py
2. See error, importlib.metadata.PackageNotFoundError: No package metadata was found for django-simple-history

Expected behavior
The tests should run without any error after following instructions in the document.