Add execution example and mention optional env

[ABaldwinHunter]

Thought this usage section would be useful to have. We don't mention the env var anywhere in README or our docs.

[maxjacobson]

We used to have more details like this, but moved it into our centralized docs: 59b4668

We could keep these details in the README as well, we just have to be conscientious about keeping everything up-to-date, which is easier when things are centralized on the docs. WDYT?

[ABaldwinHunter]

we just have to be conscientious about keeping everything up-to-date, which is easier when things are centralized on the docs. WDYT?

I think the invocation and env vars have a low enough churn that having that info in readme wouldn't hurt - and follows the strategy of a lot of other open source tools.

I also believe we don't have mention of the CODECLIMATE_API_HOST in the docs, though I imagine that's less frequently used.

Agree that moving info out of the README seems like a conscious decision though - just wanted to revisit!

[maxjacobson]

OK this LGTM - I think it's pretty clear from the structure of the document that the docs are still the place to go for beyond-basic-usage information

[maxjacobson]

@ABaldwinHunter ping

[dblandin]

CODECLIMATE_API_HOST is mentioned in the Code Climate: Enterprise docs: https://docs.codeclimate.com/docs/enterprise-setting-up-test-coverage