Update README

[aulorbe]

Problem

Our README is out of date.

Solution

Update it!

Note

• This PR includes some small code changes to naming, docstrings, etc.
• This PR includes a contributing guide as well 🎉

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [x] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

CI passes (note: some integration tests are flakey due to waiting on indexes to be ready, etc. Rerun failed runs to gauge whether failures are due to flakiness or actual problems)

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• To see the specific tasks where the Asana app for GitHub is being used, see below:
  • https://app.asana.com/0/0/1207872682458897
  • https://app.asana.com/0/0/1207879675066638
  • https://app.asana.com/0/0/1207208972408329

[austin-denoble]

We document operations using sparse values in our docstrings. So I thinkkkk it's okay for us not to include them explicitly in the README, but wdyt @austin-denoble?

I think that's fine, yes.