Update install-and-setup.md

[junaidahmed501]

Checklist

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Description

When I was adding firebase into my angular app following the exact steps that are mentioned in this repo's readme, I still had problem running my project because I somehow missed installation of firebase & it took me some time to figure it out after a lot of searching. Although it is mentioned in the readme doc file about the installation of firebase from npm, it's not too clear so that's what I've tried to add.
Apologies in advance if I've missed the guidelines but I've tried my best though.

Code sample

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[junaidahmed501]

@googlebot I signed it!

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CLAs look good, thanks!

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[jamesdaniels]

Firebase npm install is fixed with #2485, maybe we should state what the ng add command did for the developer?

[jamesdaniels]

Closing for now.