Add instructions for updating test_display images in CONTRIBUTING.md

[kianelbo]

Reference Issue

Fixes #1502

What does this implement/fix? Explain your changes.

Explains how to add or update baseline images used in tests.

Any other comments?

Is there anything to do other than copying the steps to CONTRIBUTING.md? :))

[bmcfee]

Thanks for this one! All looks good to me.

BTW in the future if you're only updating static docs and not code, you can add [ci skip] to the commit message to bypass the unit tests: https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

[codecov]

Codecov Report

Merging #1520 (fcacdb9) into main (b9830b4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1520   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          32       32           
  Lines        4151     4159    +8     
=======================================
+ Hits         4100     4108    +8     
  Misses         51       51           

Flag	Coverage Δ
unittests	98.77% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
librosa/display.py	97.00% <0.00%> (+0.06%)	⬆️

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[bmcfee]

LGTM, thanks!