Update README.md

[jonathonherbert]

What does this change?

We've a few issues at the moment detailing errors that are the result of running Typerighter in a non-Guardian context. This PR updates our readme to give a little more detail about the hurdles non-Gu devs are likely to encounter when spinning up Typerighter.

How to test

Do the new docs adequately address all the Guardian-specific infra used in running the Typerighter project?

How can we measure success?

Fewer issues raised as a result of clearer guidance for non-Guardian developers.