add an example use of firestore

[devoncarew]

• add an example use of firestore (example/example.dart)
• add an API example to the readme

The example/example.dart file shows both simple value get and set, as well as an example of the streaming listen call.

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[devoncarew]

We'll want to replace this manual routing header w/ improvements to the grpc / protobuf generator so that it adds the necessary headers.

[mosuem]

Is there a reason for downgrading?

[devoncarew]

good question; I actually forget. perhaps to retain the current (pre-3.7) formatting?

I'll keep this as is and roll separately w/ a general proto-regenerate PR.