Add SwiftPM instructions to base README.md (#6414)

* Add SwiftPM instructions to base README.md

This matches other package managers.

* Capitalized Beta

README.md

@@ -37,8 +37,9 @@ monetize your app. More information about Firebase can be found at
 
 ## Installation
 
-See the three subsections for details about three different installation methods.
+See the subsections below for details about the different installation methods.
 1. [Standard pod install](README.md#standard-pod-install)
+1. [Swift Package Manager (Beta)](SwiftPackageManager.md)
 1. [Installing from the GitHub repo](README.md#installing-from-github)
 1. [Experimental Carthage](README.md#carthage-ios-only)
 
@@ -47,6 +48,11 @@ See the three subsections for details about three different installation methods
 Go to
 [https://firebase.google.com/docs/ios/setup](https://firebase.google.com/docs/ios/setup).
 
+### Swift Package Manager (Beta)
+
+Instructions for the Beta of [Swift Package Manager](https://swift.org/package-manager/)
+support can be found at [SwiftPackageManager.md](SwiftPackageManager.md).
+
 ### Installing from GitHub
 
 For releases starting with 5.0.0, the source for each release is also deployed