Add testing/contributing instructions to README

[Mr0grog]

I finally added real tests a few months ago, but failed to document how they work and, relatedly, how fixtures work. This really needs to be clearly documented in the README (probably in a section about contributing/developing in general).

I wrote up a mini-explainer in this comment, but it doesn’t cover everything: #74 (comment)

Important points:

• e2e and unit tests
  • What belongs in each
  • How to run them (npm test vs npm run test-e2e vs npm run test-unit)
• Selecting which browsers to run using the BROWSERS environment variable.
• How Google Docs fixtures work.
  • Downloading/updating them.
  • What the deal is with <name>.copy.html vs. <name>.export.html.
  • Adding new fixtures.
  • How to use them in a test.