CONTRIBUTING.md is not complete

[tuminoid]

https://github.com/metal3-io/ironic-standalone-operator/blob/main/CONTRIBUTING.md

Requirement:

The information on how to contribute SHOULD include the requirements for acceptable contributions (e.g., a reference to any required coding standard). (URL required) [contribution_requirements]

/kind documentation

[Rozzii]

/triage accepted
/kind documentation