Update messaging.md

[Tommertom]

I think we all love Angular and if there is an Angular-ish way of using service workers together with this great AngularFire package, it should be noted here. Initially I got really stuck figuring out workbox. While it is very powerfull, but also time consuming. So I recommend giving the reader a bit more comfort about the potential/opportunities using AngularFire.

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Description

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[Tommertom]

@googlebot I signed it!

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[jamesdaniels]

Thanks, good change

[Tommertom]

@jamesdaniels I see I made typos.. Sorry about that. You can correct directly or you want me to do a PR? What is most convenient for you...

Two small typos:

• forgot closing . in - 'If you are using the Angular Service Worker, you are not currently able to use AngularFireMessaging out-of-the-box'
• double this in "Following the discussion in this [this...]

Apologies again.

[jamesdaniels]

No worries. Feel free to send a PR.