Makefile: Refer to CONTRIBUTING doc, reword test to avoid "example"

[EricCousineau-TRI]

What does this PR do?

Provides concrete example of running a single file, as is shown in other docs.

• Requires tests: Remove usage of --flake8 flag #5909 before merging

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• (N/A) Did you make sure to update the documentation with your changes? (if necessary)
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Did you have fun?

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[codecov]

Codecov Report

Merging #5910 (a37c60c) into master (b5d29df) will decrease coverage by 42%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #5910     +/-   ##
========================================
- Coverage      88%     46%    -42%     
========================================
  Files         170     170             
  Lines       11805   11648    -157     
========================================
- Hits        10399    5336   -5063     
- Misses       1406    6312   +4906     

[Borda]

I think that we do not need it here makefile anymore...
just if it is missing add it to the contribution guide

[EricCousineau-TRI]

Done! There's already a mention of how to do this in the section for adding a new test, but I've also added a concrete example for an existing test.

[Borda]

@EricCousineau-TRI thank you for sending your PR, just a minor issue coming from our side... we have swapped branches regarding upcoming feat 1.2, mind rebase on actual master? if needed see How to fix PR with mixed base and target branches?

[EricCousineau-TRI]

Sounds good! I'll wait for #5909 to land before rebasing.

[Borda]

Sounds good! I'll wait for #5909 to land before rebasing.

hi @EricCousineau-TRI your mentioned PR is merged, let's get this done too :]

[EricCousineau-TRI]

Done!