Initial governance based on MVG

[iamwillbar]

This is an initial set of governance documents for the package-url organization, based on:
https://github.blog/2021-07-22-minimum-viable-governance-lightweight-community-structure-foss-projects/

Other than the "Mission" in CHARTER.md (which was adapted from the README in the purl-spec repository) the content is directly from https://github.com/github/MVG.

Open issues:

• Choose a contact method for Code of Conduct issues
• Determine if we should remove trademark content since the project does not currently have any registered trademarks
• Determine who the initial steering committee members are (proposal: the current people with "Owner" permissions on the package-url organization form the initial steering committee)
• Steering committee members "sign" by adding themselves to the STEERING-COMMITTEE.md file

Next steps:
@pombredanne as the founder of the project, can you decide whether the above proposal for initial steering committee members is acceptable to you? If it is, I will convert this from a Draft pull request and assign to the initial steering committee members for review and "signature".

[iamwillbar]

@pombredanne it would be great to get your thoughts on this before I open up to review by the other maintainers.