add warning to docs about renaming superclass

[JohananOppongAmoateng]

Added a warning to the docs warning users about renaming superclass which causes migration problems

Closes #396

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.91%. Comparing base (139db51) to head (7c377e4).
⚠️ Report is 24 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #764   +/-   ##
=======================================
  Coverage   82.91%   82.91%           
=======================================
  Files          26       26           
  Lines        1580     1580           
  Branches      250      250           
=======================================
  Hits         1310     1310           
  Misses        206      206           
  Partials       64       64           

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[bckohan]

This is an upstream issue with Django, outside of our purview to document or fix. I converted #396 to Q&A and answered it.