Document how to set up a production-like environment

[equivalentideas]

This is extracted from https://github.com/openaustralia/morph/pull/1156

@auxesis has suggested this would make it much easier for people to contribute, to set up their own morph, and for our team to develop.

@auxesis if you could describe a little what's missing I think that would be a good start here.

[auxesis]

At a very minimum, we need documentation on:

• [ ] How to set up a production-like environment (on something like Linode or Digital Ocean)
• [ ] How to set up a local development environment (with something like Vagrant or Docker)
• [ ] How to manage credentials in production
• [ ] How to manage credentials in development

Adding a full guide like this to the README is going to make it way too long, so I think we should split it out into separate docs under docs/.

[KrzysztofMadejski]

My 2 cents: The development part could land in CONTRIBUTE.md