Added README to base-template and tweaked chunked list advice

[camerons]

Purpose / why

• README added based on feedback that it wasn't clear where to start with templates.
• chunked list advice is review of aidantwork

Verification

• Read new text, confirm it improves our story.
• This is not addressing an issue as I'm not using issues in the incubator during this development phase.

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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?

[aidanatwork]

@camerons, I made a couple minor suggestions for changes, for improved readability.