Documented how to add DJA to installed apps

[sliverc]

Description of the Change

This setting is now needed for browsable api and openapi schema. There might be more features coming needing this and it is also common practice to do this for django apps.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #963 (8c1a624) into master (4c62f1d) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #963      +/-   ##
==========================================
- Coverage   96.82%   96.78%   -0.04%     
==========================================
  Files          65       65              
  Lines        3904     3857      -47     
==========================================
- Hits         3780     3733      -47     
  Misses        124      124              

Impacted Files	Coverage Δ
example/serializers.py	88.82% <0.00%> (-0.32%)	⬇️
rest_framework_json_api/renderers.py	89.96% <0.00%> (-0.20%)	⬇️
rest_framework_json_api/relations.py	88.34% <0.00%> (-0.12%)	⬇️
rest_framework_json_api/parsers.py	98.48% <0.00%> (-0.07%)	⬇️
tests/conftest.py	100.00% <0.00%> (ø)
tests/test_views.py	100.00% <0.00%> (ø)
example/factories.py	100.00% <0.00%> (ø)
example/tests/test_errors.py	100.00% <0.00%> (ø)
example/tests/test_openapi.py	100.00% <0.00%> (ø)
example/api/resources/identity.py	100.00% <0.00%> (ø)
... and 8 more

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