Add to the README the Passwordless email variables that are available

[willryanuk]

When configuring the extension, it isn't clear what variables are available for the login email.

I think the only thing we have is:
<%= URL %>?loginToken=<%= CODE %>
... for generating the login URL.

I'd actually like to know if we can alter the messaging for NEW USERS vs RETURNING USERS. And also, if there is any possibility for control logic in the email? Can the subject contain any variables/ control logic?

[OlesCrown]

Also, it would be nice to have a LANGUAGE SELECTOR for the emails. We can add an additional language tag in the context when submitting the request.