docs: Update README with additional code guidelines on models and django signals

[matthewelwell]

Added code guidelines for models and asynchronous dispatch.

Thanks for submitting a PR! Please check the boxes below:

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

To continue the discussion from engineering meeting on 8th July 2026.

How did you test this code?

n/a

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3 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
docs	Ignored		Jul 8, 2026 12:51pm
flagsmith-frontend-preview	Ignored		Jul 8, 2026 12:51pm
flagsmith-frontend-staging	Ignored		Jul 8, 2026 12:51pm

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.63%. Comparing base (aa4ad62) to head (2fdb03c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7967   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files        1497     1497           
  Lines       59192    59192           
=======================================
  Hits        58386    58386           
  Misses        806      806           

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