Update README

[emmyoop]

@leahwicz and I decided that this repo should be owned by the Core Group at dbt Labs. We are currently the only team using the actions within the repo. We need to establish expectations around using an developing in this repo.

• Get Admin access to the repo
• Add this as a repo for the Core Group
• Update README to indicate the Core Group owns this repo
  • add standards (ie, always point to main branch, no tags)
  • backwards compatibility
• Add Jira hooks to auto-generate jira issues 😅