Add details about Google billing accounts for API keys

[lognaturel]

The Google maps API key process has changed in the last few months. I also added an explanation for why the keys that are in the source code 1) don't work for debug builds 2) are ok to leave in unprotected.

What has been done to verify that this works as intended?

I have tried the instructions and successfully generated a Google Maps API key. I read through the terms and confirmed that a trial credit is included and that the card won't be billed without a confirmation.

Why is this the best possible solution? Were any other approaches considered?

It's a matter-of-fact description of the current state of things. I considered bolding or italicizing parts of the message but thought that was too aggressive.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

It warns users ahead of time about the billing process so they won't try to get a key if they don't have a credit card, for example. It's a little scary/offputting but I don't think we have a choice if we keep using the Google Maps API.

Do we need any specific form for testing your changes? If so, please attach one.

No

Does this change require updates to documentation? If so, please file an issue here and include the link below.

This is it!

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