Add information about individual @NgModules

[fisherds]

Copied some material from #854 into this doc

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Description

Code sample

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[davideast]

Love this change! Just a small bit about clarifying the NgModule imports.

[davideast]

Add a comment for each line saying what it adds in terms of Firebase, just so we can help those that hastily copy and paste.

AngularFireModule.initializeApp(firebaseConfig, 'my-app-name'), // imports firebase/app needed for everything
AngularFireDatabaseModule, // imports firebase/database, only needed for database features
AngularFireAuthModule, // imports firebase/auth, only needed for auth features

[fisherds]

Love the additions Dave. I signed the CLA btw. I also used to work at Google so hopefully I'm trustworthy enough. ;)

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[davideast]

Thanks @fisherds!