Write development docs

[snakescott]

I would like to poke around to see if I can improve some ncluster testing but there are no docs on development (how to run existing tests, how CI works, etc).

[yaroslavvb]

That's because there are no tests and no CI :)

The most important test I think is ensuring that "new user" experience isn't broken -- making sure a first time user with fresh AWS account and no infrastructure can do ncluster.make_task() and things work out of the box.

I think this will require live AWS testing -- I had to rely on some undocumented AWS features which may stop working when AWS changes their ways. For example, setting up machine requires login, but AWS doesn't provide a direct way to tell whether DLAMI login name is ec2-user@, or ubuntu@, so this has to be inferred indirectly from heuristics like image.root_device_name or AMI description.

My current idea is to setup CircleCI pipeline which will run this on a testing AWS account and log results into wandb dashboard.