Add SETTINGS section to the README

[thenewguy]

It would be nice if all of the available settings were listed together in the README. I only found CUCUMBER_SES_REGION_NAME and its default of AWS_SES_REGION_NAME because I checked closed tickets before submitting one and saw https://github.com/duointeractive/sea-cucumber/pull/16/files

If there is already a list somewhere, I apologize. I read through everything I could find, but I was in a hurry.

[gtaylor]

I'd be more than glad to accept a pull request for this. We're not actually using sea-cucumber anymore, so this would probably be the only way it'd get done.

[thenewguy]

Sure, no problem.

[elchiapp]

Fix: #26 .

[debuggerpk]

@gtaylor i am interested, what are you guys using then?