Update README with some basic run / deploy commands

[wstrange]

As a nodejs newbie, I'm a little lost on the run / deployment model.

a) How do I run this with node?
b) Does this work with the firestore emulator? Any tips?

[wstrange]

Doing some experimenting, this is what worked for me:

To initialize the app, the following is sufficient:

final admin = FirebaseAdmin.instance;
final app = admin.initializeApp();

Note that it is not required to pass in the service account json, or the URL of the emulator, provided you do the following:

To run against the "real" firestore, set the following env var:

export  GOOGLE_APPLICATION_CREDENTIALS=path/to/your/service_account.json
# Run your compiled app
node build/node/my_admin_app.dart.js

To run against the emulator (assume it is running firebase emulators:start), make sure the previous variable is unset, and intead set:

export FIRESTORE_EMULATOR_HOST=localhost:8080
node build/node/my_admin_app.dart.js