docs: mention upgrade notes

[belgattitude]

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Latest commit: e55827f

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a5dd98) 93.13% compared to head (e55827f) 92.90%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #874      +/-   ##
==========================================
- Coverage   93.13%   92.90%   -0.23%     
==========================================
  Files          96       96              
  Lines         670      677       +7     
  Branches      188      193       +5     
==========================================
+ Hits          624      629       +5     
- Misses         40       42       +2     
  Partials        6        6              

Flag	Coverage Δ
httpx-assert-unit	91.47% <ø> (ø)
httpx-dsn-parser-unit	100.00% <ø> (ø)
httpx-exception-unit	95.51% <ø> (-0.49%)	⬇️
httpx-json-api-unit	65.30% <ø> (ø)

see 4 files with indirect coverage changes