Document usage of Angularfire dev dependencies

[KingDarBoja]

Should be a good idea to document the following dependencies being added by the ng-add schematic:

• fuzzy
• inquirer
• inquirer-autocomplete-prompt
• open

In this case, only open is being used for the ng deploy --preview command (as stated in the Angular discord firebase channel) whereas the others are needed only in the ng add step, so those can be safely removed after it 🤔

[google-oss-bot]

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[jamesdaniels]

FWIW open and inquirer, maybe even fuzzy / autocomplete, are likely already in your node_modules between firebase-tools and ng-cli. I’m adding them to devDeps to be defensive.

[jamesdaniels]

Dropped dev dependencies in 7.1.0-rc.0