Update README.md

[katowulf]

Add link to AngularFire2.

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[katowulf]

I signed it! For maybe the 100th time. Really.

[jwngr]

Closed in favor or #739.

[coveralls]

Coverage remained the same at 93.886% when pulling 1d7f0b2 on katowulf-af2-readme into afa0788 on master.