Update README.md

[wanderingstan]

Looks like firebase now requires logging in and an initialization step. (Probably would be good to indicate what options to select when initializing too!)

➜ firebase use --add

Error: Failed to authenticate, have you run firebase login?

➜ firebase use --add

Error: firebase use must be run from a Firebase project directory.

Run firebase init to start a project directory in the current folder.

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[jhuleatt]

Thanks for the documentation improvement @wanderingstan! I can merge this once you sign the CLA