Update CONTRIBUTING.md - Added Verified Commits as a Requirement - Closes#445

[alien299]

Relevant components:

• [ ] Signalling server
• [ ] Frontend library
• [ ] Frontend UI library
• [ ] Matchmaker
• [ ] Platform scripts
• [ ] SFU

Problem statement:

Verified commits are a requirement for Pull Requests but it isn't stated in the Contribution Guidelines. Issue #445

Solution

Extended the Pull Request part with signing commits. #445

[mcottontensor]

Thank you for this.