Update documentation to describe how use it with SPM

[GabriellCosta]

Step 2: Describe your environment

• Objective C or Swift: Swift
• iOS version: 15
• Firebase SDK version: 10.15.0
• FirebaseUI version: 13.0.0
• CocoaPods Version: ____

Step 3: Describe the problem:

There's no documentation on how to setup Firebase UI with SPM

Steps to reproduce:

1. Reading the documentation
2. There's no information

Observed Results:

• What happened? This could be a description, log output, etc.

Expected Results:

• What did you expect to happen?
• Read in the Readme in the installation part how to set a project and a framework using SPM

As part of this, I already have done the configuration and I'm ready to open a PR with the updates for the README

[morganchen12]

If you have the changes ready on a branch, feel free to make a PR and I'll review it. Otherwise I can spend some time this week writing installation instructions.

[ZMonk91]

It should be noted, firebaseui-ios depends on googlesignin being 6.0.0 as well.

[ningzhiqun]

have document ？