Add contributing guidelines

[pjbull]

First pass at contributing guidelines and a code of conduct in case folks from hacktoberfest come through.

Closes #213

[github-actions]

🚀 Deployed on https://deploy-preview-367--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

Merging #367 (015d4c7) into master (703607c) will decrease coverage by 0.9%.
The diff coverage is n/a.

❗ Current head 015d4c7 differs from pull request most recent head 2c9724c. Consider uploading reports for the commit 2c9724c to get more accurate results

Additional details and impacted files

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- Hits         1465    1452     -13     
- Misses         98     111     +13     

see 3 files with indirect coverage changes

[klwetstone]

@pjbull These are very clear and extremely well written! Easy to read, includes all the details needed, and with exactly the right level of detail. My comments are generally about page flow and organization.

I'd recommend making a clearer distinction between key steps needed for any PR vs. general tips about working the repo, each of which which may only be relevant to some PRs. I would add a section at the top (right under the bulleted "first, a few guidelines") that's something like:

How to contribute

0. File an issue. No PRs from outside contributors are accepted without an issue. We respect your time and want to make sure any work you do will be reviewed, so please wait for a maintainer to sign off before getting started.
1. Create a local environment.
2. Make changes in your local version of the repository.
3. Make sure that the tests pass locally.
4. Update the package documentation, if applicable.
5. Go through the items in the final PR checklist.
6. Submit a PR!

For some guidance on working with the code, see the sections on [code standards](link to new section described below) and code architecture.

The content is GREAT -- comprehensive, easy to understand, and thorough. To make it easier to find the right pieces of info, I would reorganize the sections to match the order of the new "how to contribute" bullets, and to separate out steps that are always necessary from tips/tricks that may not be relevant to every PR. My suggested outline would be:

## Local development
- Just instructions to create an environment - what's currently under "local development but before "adding dependencies"
## Tests
- Same sections as before, minus interactive testing
- Change each sub-header to be "###" instead of "####"
## Documentation
- Same sections as before
- Change each sub-header to be "###" instead of "####"
## Submitting a PR (same sections as before)
## Code standards and tips
  ### Adding dependencies
  ### Linting and formatting
  ### Type hinting
  ### Credentials and cloud access
  ### Interactive testing (moved from the section on "tests")
  ### Exceptions (moved from "code architecture")
## Code architecture (same sections as before minus exceptions)
## Governance