Update CONTRIBUTING.md to include building JS

[sunboyy]

This pull request updates CONTRIBUTING.md in the Building Gitea section. I added information about building JavaScript files so that new contributors can setup the project more easily.

[codecov-io]

Codecov Report

Merging #9454 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9454      +/-   ##
==========================================
- Coverage   41.51%   41.51%   -0.01%     
==========================================
  Files         571      571              
  Lines       74648    74648              
==========================================
- Hits        30993    30988       -5     
- Misses      39815    39820       +5     
  Partials     3840     3840

Impacted Files	Coverage Δ
models/unit.go	62.16% <0%> (-5.41%)	⬇️
routers/repo/view.go	38.59% <0%> (-0.88%)	⬇️
models/gpg_key.go	55.03% <0%> (-0.56%)	⬇️
models/repo_list.go	74.07% <0%> (+0.92%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️

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[zeripath]

I think this is out-of-date make build will generate the css and js now.

[lunny]

You can still use make generate build but generate could be ignored.

dismissing my review

[6543]

ping lgtm