Add label management instructions and default labels file

[hillaryj]

🗣 Description

Adds a how-to guide for standardizing labels across the organization and a file containing defaults.

Closes https://github.com/cisagov/.github/issues/7

💭 Motivation and Context

Adds documentation for the label-standardization process plus default labels so we can do this again if we need to.

🧪 Testing

Linting passes; labels were successfully applied as per https://github.com/cisagov/.github/issues/7

✅ Checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] All new and existing tests pass.

[hillaryj]

After discussing with @mcdonnnj and for ease of future use, I've created https://github.com/cisagov/github-label-management which will host an updated and genericized version of this tool. I'm incorporating the feedback here into that project and will close this PR once I've gotten that up and running.

[jsf9k]

@cisagov/team-ois, can we close this ancient PR?

[mcdonnnj]

@cisagov/team-ois, can we close this ancient PR?

Shut 'er down. The repository reference in https://github.com/cisagov/development-guide/pull/41#issuecomment-748163077 has already been deleted as well.

[jsf9k]

Closing this ancient PR, as we already have automated label management via a GitHub Action inherited from cisagov/skeleton-generic.