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

[muneeb706]

Description

Added pip install django-simple-history command under Testing section of Contributing.rst file.

Related Issue

#1293

Motivation and Context

Reader of the document should be able to run tests successfully after going through the instructions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f48b53d) 96.87% compared to head (574773b) 96.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
- Coverage   96.87%   96.79%   -0.08%     
==========================================
  Files          23       23              
  Lines        1278     1278              
  Branches      211      211              
==========================================
- Hits         1238     1237       -1     
  Misses         21       21              
- Partials       19       20       +1     

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