Update contributing documentation for local backends

[Skelmis]

The backend requires the project to be built for dist files to be in place. This PR just makes that a touch clearer

[dantownsend]

Thanks for this. The PR is failing due to some unrelated problems with the CI. I'll fix it, then merge in this branch.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.02%. Comparing base (1157c12) to head (ed010e4).
Report is 31 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   93.42%   94.02%   +0.60%     
==========================================
  Files           5        5              
  Lines         365      402      +37     
==========================================
+ Hits          341      378      +37     
  Misses         24       24              

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