CocoaPods Installation

On the README, the instructions to install Firebase SDK are:

```
cd ios && pod init
pod install Firebase/Messaging
```

From the [official instructions](https://firebase.google.com/docs/ios/setup), the `Firebase/Core` is also needed.

Additionally, running `pod install Firebase/Messaging` didn't worked for me. Instead, I created a Podfile containing:

```
pod 'Firebase/Core'
pod 'Firebase/Messaging'
```

After that, running `pod install` was enough to download the dependencies.

Another note that newbies on IOS/CocoaPod may encounter (I spent some hours on this) was that you need to open the `yourpoject.xcworkspace` instead of `yourproject.xcodeproj`.

I'm not sure if any of these informations are specific on my setup, so I won't submit a PR for the README, but I think it's worth adding more information on it.

Start in the README’s Firebase SDK installation instructions and compare them with the linked official Firebase iOS setup guidance. Verify the Firebase/Core and Firebase/Messaging Podfile steps and the workspace-versus-project opening note; the README should give working, unambiguous setup instructions for newcomers.

Written by the indexing model from the issue text.