Update the README.md so it's clear how to set up the Google API key

[jenlampton]

As per #9

It appears that you need to have a billing account with Google and linked to to the project as well. You get a credit limit so, at least for smaller websites, you'll likely still not to pay for using Google Maps and Geocoding in your site.

[laryn]

I'm not complete yet but did some minor cleanup while trying to troubleshoot. The biggest issue may be that the details of the error message were not displaying (which makes it hard to troubleshoot).

WIP PR: #21

At the moment the status I'm getting is "API keys with referer restrictions cannot be used with this API." but I don't have direct access to the API Key settings so can't change it -- will come back to it tomorrow and see if I get it working. If so I'll try to update the README as part of the PR above.

[jenlampton]

Since this PR is mostly text changes to the interface, I'm going to merge it for the next release, but I'll leave this issue open so we can update the README.