Add Release Checklist

[cmsirbu]

This PR documents, step by step, what a maintainer should do when cutting a new release for an open source App.

• Towncrier checks now disabled on any release-related branches and PRs
• Updated towncrier template to include the relevant major.minor
• Fixed #115

I have tested these changes successfully in the https://github.com/nautobot/nautobot-app-dev-example repo, with the exception of the LTM steps, due to a lack of an LTM branch in dev-example.

[smk4664]

Just because I hit this today, the Update Documentation section should include a note about updating the Compatibility Matrix.