Update CONTRIBUTING.md

[amyschoen]

Proposed changes

Updating to reflect new CLA signing process

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (changes to code, which do not change application behavior)

Checklist

• I have filled out this PR template
• I have read the CONTRIBUTING doc
• I have added automated tests that prove my fix is effective or that my feature works
• I have added necessary documentation (README.md, inline comments, etc.)
• I have updated the CHANGELOG.md under a ## next release, with a short summary of my changes

Relevant Links

Further comments

[CLAassistant]

All committers have signed the CLA.

[amyschoen]

This PR has been ignored for far too long, and the new contribution process has been our standard since August. I'm overriding and merging this in.