Add some docs for allowing sign up

[jagthedrummer]

Fixes #103

[paulcwatts]

Thanks for taking the lead on this! Now that I have a look at it, it does seem like a lot of information to squeeze into the README. Perhaps add a guides/ directory with a signup.md file within it. In the README you can have a section called "Other Guides" with a link to this new file.

You can use this as inspiration:
https://github.com/simplabs/ember-simple-auth/blob/master/README.md#other-guides
https://github.com/simplabs/ember-simple-auth/blob/master/guides/managing-current-user.md

In the future, it's possible even some more of the README could be moved into that directory, like the Advanced Configuration section.