Add easy to run couchdb server with Docker and Fig

I added some instructions and a fig.yml file to help users get started on testing
pouchdb changes.

I feel like a docker based approach is the fastest and most accurate way. Because now the tested couchdb server version is written in the fig.yml file.

With this approach you could even have multi couchdb version testing easily.

If you want to go further that way let me know.

update CONTRIBUTING and TESTING accordingly
remove test part from CONTRIBUTING, link to TESTING, DRY

What do you think?