Document example for overrides (i.e. allow_failure)

I recently enabled salsa-ci on an existing project and of course not all of the tests succeed (i.e. it is currently not reproducible). I don't want to completely skip the test, because it is a goal and I want to know whether I came closer, but it should not mark the commit as failed.

In the end it was pretty easy

include: 
 - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/salsa-ci.yml
 - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/pipeline-jobs.yml

reprotest:
  allow_failure: true

maybe this could be documented in the README as an option to skipping jobs

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