Update CONTRIBUTING.md

[kwk]

Add note on how to treat PR branches.

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@@            Coverage Diff            @@
##             master    #2367   +/-   ##
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  Coverage          ?   69.85%           
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  Files             ?      171           
  Lines             ?    16976           
  Branches          ?        0           
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  Hits              ?    11859           
  Misses            ?     3951           
  Partials          ?     1166

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[corinnekrych]

@kwk I think having a how to guide would be more specific, here is a proposal: #2369