Authentication Documentation is Out of Date

[danielburn5]

The authentication section of the README points the reader to operationcode.org/join in order to register and get themselves a password for local development. However, this process seems to be out of date as it's not possible to do this through the join page. Doing so results in a slack invite only. Also, there is no login page on the site anymore, from what I can see.

Docs should be updated with whatever the current process is for authentication in a development environment.

It looks like there is a set of credentials in the test setup from a cursory glance, but this needs confirming. If so, maybe we could utilise them for local dev so long as there are no security implications.