Improve contributing instructions

[tianchu]

What does this PR do?

• Improve contributing instructions
• Add the slack community link that allows self-invitation.

Motivation

Testing Guidelines

Additional Notes

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Checklist

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-io]

Codecov Report

Merging #56 (d643d90) into main (c3e0b77) will decrease coverage by 0.32%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   82.89%   82.56%   -0.33%     
==========================================
  Files          12       12              
  Lines         608      608              
==========================================
- Hits          504      502       -2     
- Misses         81       83       +2     
  Partials       23       23              

Impacted Files	Coverage Δ
internal/metrics/processor.go	93.75% <0.00%> (-3.13%)	:arrow_down:

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