Improve README to make it easier to set Maptio up locally

[rgoj]

Now that we've gone open source, it'd be great to move the README from being an internal document to something that can really be used to get Maptio set up without much difficulty.

This will be about just trying to start from scratch - without access to the internal environment variables file - and seeing what is essential to getting up and running locally, which external services need to be set up, etc.

We've had a couple of bits of feedback about it already, so making a note of this here (internal links):

• https://app.intercom.com/a/apps/q3x5lnhp/inbox/inbox/all/conversations/106323200002082
• https://mail.google.com/mail/u/0/#search/stuart/KtbxLxGvdtthzfqGqPgxZBDLJzgCTssKdB

As this is essentially an infinite project - i.e. I could think of things to improve the docs to occupy me for years :) - maybe it'd be worth treating this issue as an initial timebox - get as far as we can in a day or so, perhaps.

[rgoj]

Also see to #811