docs: add developer.md

[APN-Pucky]

Closes: #177

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.87%. Comparing base (d4231f5) to head (28b9cb0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #266   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files           2        2           
  Lines         320      320           
  Branches       64       64           
=======================================
  Hits          294      294           
  Misses         17       17           
  Partials        9        9           

Flag	Coverage Δ
unittests-3.10	91.87% <ø> (ø)
unittests-3.11	91.87% <ø> (ø)
unittests-3.12	91.87% <ø> (ø)
unittests-3.13	91.87% <ø> (ø)
unittests-3.8	91.87% <ø> (ø)
unittests-3.9	91.87% <ø> (ø)

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[APN-Pucky]

@matthewfeickert I am trying to create a 0.9.1 version and I followed your video, but https://github.com/scikit-hep/pylhe/actions/runs/11403842254/job/31731751820#step:8:46 here "Bump version" fails with

fatal: Failed to resolve 'v0.9.1^{}' as a valid ref.

Further I still need to figure out conda after pypi.