Update README with xcodebuild example.

[niilohlin]

This documents the question in #2398.

[codecov-io]

Codecov Report

Merging #2400 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2400   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files         319      319           
  Lines       16154    16154           
=======================================
  Hits        14666    14666           
  Misses       1488     1488

Impacted Files	Coverage Δ
Source/SwiftLintFramework/Models/Command.swift	97.4% <0%> (-1.3%)	⬇️
...tFramework/Rules/Lint/ValidIBInspectableRule.swift	100% <0%> (+1.36%)	⬆️

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[marcelofabri]

Thanks!