Added Natural Key Serialization section to docs

[JohananOppongAmoateng]

When using Django's natural key serialization with :django-admin:dumpdata and
:django-admin:loaddata, polymorphic models require special handling.
polymorphic.managers.PolymorphicQuerySet.non_polymorphic in
get_by_natural_key() for polymorphic models. Without this, deserialization fails
when loading new objects because polymorphic queries try to fetch incomplete objects.

Closes #517

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.44%. Comparing base (0d30c86) to head (692eb37).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #790   +/-   ##
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  Coverage   83.44%   83.44%           
=======================================
  Files          26       26           
  Lines        1649     1649           
  Branches      264      264           
=======================================
  Hits         1376     1376           
  Misses        207      207           
  Partials       66       66           

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