README: Explain how to build and display documentation locally

[melvo]

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/xep.doc
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #453 (74cc776) into master (74cc776) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 74cc776 differs from pull request most recent head a269173. Consider uploading reports for the commit a269173 to get more accurate results

@@           Coverage Diff           @@
##           master     #453   +/-   ##
=======================================
  Coverage   68.56%   68.56%           
=======================================
  Files         267      267           
  Lines       23607    23607           
=======================================
  Hits        16187    16187           
  Misses       7420     7420           

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[melvo]

This is closed in favor of #539.