Add section about customizing mail

[strk]

See #6037

[codecov-io]

Codecov Report

Merging #7419 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7419      +/-   ##
==========================================
+ Coverage   41.18%   41.19%   +<.01%     
==========================================
  Files         469      469              
  Lines       63536    63536              
==========================================
+ Hits        26167    26171       +4     
+ Misses      33948    33943       -5     
- Partials     3421     3422       +1

Impacted Files	Coverage Δ
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️
routers/repo/view.go	43.25% <0%> (+1.01%)	⬆️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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