chore(docs) Update CONTRIBUTING with npm pack

[rwaskiewicz]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe): Backport of fix(testing): puppeteer v10 support #2934

What is the current behavior?

We don't discuss testing with npm pack in CONTRIBUTING.md, and can potentially commit the created tarball to git 🤢

What is the new behavior?

• add lessons learned from using npm pack to the CONTRIBUTING.md doc
• update .gitignore to ignote .tgz files

Does this introduce a breaking change?

• Yes
• No

Other information

• I plan on squashing this branch when it's time to merge. I'll update the commit body at that time