Document procedure for seting up SES on the dev awsbox.

This adds postfix to the default awsbox setup, along with some placeholders to ease configuration of the proper SES credentials.  I've documented the necessary steps for setting up SES in the README.  Suggestions for how to manage that in a more automated fashion would be most welcome.

Fixes https://github.com/mozilla/picl-idp/issues/161

@zaach r?