Update documentation

[karthik]

Two issues. There is little documentation on setting up a new application, which do_auth() requires to have a client_id. Also while the client is quite informative (digocean_client_id) the key is a very generic variable name (api_key), which can lead to confusion with other packages.

Also instead of interactively requiring keys (when missing), how about authorizing over the web and writing the credentials to a new env?

[sckott]

Right, i assume we would do auth over web, but docs say they just want the oauth token https://developers.digitalocean.com/v2/#authentication

[sckott]

@karthik good point about better docs though.

[karthik]

Sorry the comments were on the wrong version. A more streamlined auth would be great.

[sckott]

done, addressed in #20