Documentation for ECS/Fargate

[jtatum]

As it turns out, you can run Sendy in ECS, and even on Fargate. Using an EFS volume to persist uploads (see #9), Sendy will happily run on AWS. It might be nice to document a sample ECS task definition.

[jtatum]

Added some notes at https://gist.github.com/jtatum/15994aabd469cd9120ceb2a45b509b8c