Instructions for the release process

[dbeatty10]

This is a:

• [x] documentation update

Related to

• https://github.com/dbt-labs/dbt-utils/pull/821
• https://github.com/dbt-labs/dbt-codegen/pull/137
• https://github.com/dbt-labs/dbt-audit-helper/pull/80
• https://github.com/dbt-labs/dbt-external-tables/pull/227

Description & motivation

This PR add a RELEASE.md file so that it has instructions for the release process.

Checklist

• [x] This code is associated with an Issue which has been triaged and accepted for development.