Document usage of pifpaf for manual builds

The README mentions that building everything via tox uses pifpaf to run the required postgresql database. It feels like a good idea to also add pifpaf to the command lines required for the manual builds.

assigned to @lunar

Jenkins job DDOC/builds #141 failed .
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LGTM, but you need to remove that XXX commit

approved this merge request

added 5 commits

• 126bf0b0...895f2092 - 4 commits from branch swh/devel:master
• c6e9f5dc - Document usage of pifpaf for manual builds

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enabled an automatic merge when the pipeline for c6e9f5dc succeeds

Jenkins job DDOC/builds #165 succeeded .
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merged