merge contrib guide from incubator

[Loquacity]

ALL Pull-Requests require an associated Issue as documented in How to contribute.

Resolves: #171

Purpose / why

We have several locations with Contributors' Guides, this PR aligns the content in a single place.

What changes were made?

• Content copied from Incubator repo base-template/contributors-guide.md to Template repo CONTRIBUTING.md
• Some light editing to ensure the content flows correctly
• Moved CONTRIBUTING.md to .github/CONTRIBUTING.md so that it appears in the Insights tab and is inherited across all the project repos.

Still to do:

• When this PR merges, delete the contributors' guide content from the Incubator repo. This will happen when Base template - draft #2 incubator#14 merges.

Verification

• Review the content of the guide.
• Ensure it is in the correct location.

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Checklist

Pull-request reviewer should ensure the following

• Are issues linked correctly?
• Is this PR labeled correctly?
• If template updates: do they align with developers.google.com/style/?
• Did the PR receive at least one 👍 and no 👎 from core-maintainers?
• On merging, did you complete the merge using keywords?
• On merging, did you add any applicable notes to a draft release and link to this PR?

[barbaricyawps]

Looks good! Just one minor issue!