Add missing repository setting docs in cheat sheet

[strk]

Taken from app.ini-sample

[strk]

I started this to document PREFERRED_LICENSES but I don't really see that variable used in code, why was in app.ini-sample at all ?

[codecov-io]

Codecov Report

Merging #3230 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #3230      +/-   ##
=========================================
+ Coverage   34.86%   34.9%   +0.04%     
=========================================
  Files         277     277              
  Lines       40014   40014              
=========================================
+ Hits        13951   13968      +17     
+ Misses      24040   24023      -17     
  Partials     2023    2023

Impacted Files	Coverage Δ
models/repo.go	41.47% <0%> (+0.18%)	⬆️
models/repo_indexer.go	53.88% <0%> (+2.91%)	⬆️
modules/indexer/repo.go	67.82% <0%> (+6.95%)	⬆️

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[lafriks]

@strk it is used in here:

gitea/models/repo.go

Line 117 in f2e20c8

for _, name := range setting.Repository.PreferredLicenses {

[lafriks]

LGTM

[sapk]

LGTM