Add cookbook entry for user charges

[IAlibay]

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (181afab) 93.10% compared to head (f95e75d) 91.37%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
...tests/protocols/test_openmm_equil_rfe_protocols.py	78.43%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #687      +/-   ##
==========================================
- Coverage   93.10%   91.37%   -1.74%     
==========================================
  Files         132      132              
  Lines        9109     9158      +49     
==========================================
- Hits         8481     8368     -113     
- Misses        628      790     +162     

Flag	Coverage Δ
fast-tests	91.37% <85.13%> (?)
slow-tests	?

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[dwhswenson]

LGTM. It works; the notebook is there in the RTD PR build. I don't know why CodeCov is complaining, but clearly it is about stuff that isn't actually in this diff.