Update contributing guide

[edenlightning]

What does this PR do?

Fixes # (issue)

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together? Otherwise, we ask you to create a separate PR for every change.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

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[codecov]

Codecov Report

Merging #2830 into master will decrease coverage by 8%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2830     +/-   ##
========================================
- Coverage      90%     82%     -8%     
========================================
  Files          78      79      +1     
  Lines        6921    8231   +1310     
========================================
+ Hits         6238    6734    +496     
- Misses        683    1497    +814