[tor-bugs] #25388 [Core Tor/Tor]: document how to allow partially failing builds in Travis CI

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#25388: document how to allow partially failing builds in Travis CI
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     Reporter:  catalyst      |      Owner:  catalyst
         Type:  defect        |     Status:  assigned
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-ci
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Sometimes configurations that fewer developers regularly build with get
 merged with changes that break our Travis CI builds.

 We should make it easier to temporarily allow those builds to fail while
 people work on getting them fixed, because sometimes it takes a while.
 This can include adding commented-out `allow_failure` clauses.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25388>
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