Improve Docs + setup CI tests

[mbeilin]

Improve Docs + setup CI tests.

• I expect to see a README update documenting this for:
  • users (i.e. those using the API)
  • admins installing this (e.g. can i configure the point at which i switch from streaming to bulk export)
  • some info for developers on design approach
• I'd expect a test of the lower level functionality like extracting a big query query to a file in gcs (do we have that? if not let's add something for that helper method).
• Do we CI tests? if so can we get badge into the README.

Originally posted by @rufuspollock in #2 (comment)

[rufuspollock]

@mbeilin please can we prioritize this asap as needed for the JS integration work.

[rufuspollock]

@mbeilin any update here?