docs: Document Content-Digest body handling and resource limits

[dadrus]

This PR updates the README documentation around content-digest handling.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.52%. Comparing base (ebd49e0) to head (5e1c415).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   94.52%   94.52%           
=======================================
  Files          18       18           
  Lines        1315     1315           
=======================================
  Hits         1243     1243           
  Misses         37       37           
  Partials       35       35           

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