Initial SPM instructions for Firebase itself (#7605)

Author: paulb777

README.md

@@ -104,6 +104,13 @@ To develop Firebase software in this repository, ensure that you have at least
 the following software:
 
   * Xcode 12.0 (or later)
+
+CocoaPods is still the canonical way to develop, but much of the repo now supports
+development with Swift Package Manager.
+
+### CocoaPods
+
+Install
   * CocoaPods 1.10.0 (or later)
   * [CocoaPods generate](https://github.com/square/cocoapods-generate)
 
@@ -121,7 +128,7 @@ CocoaPods workspaces.
 Firestore has a self contained Xcode project. See
 [Firestore/README.md](Firestore/README.md).
 
-### Development for Catalyst
+#### Development for Catalyst
 * `pod gen {name here}.podspec --local-sources=./ --auto-open --platforms=ios`
 * Check the Mac box in the App-iOS Build Settings
 * Sign the App in the Settings Signing & Capabilities tab
@@ -136,6 +143,12 @@ Alternatively disable signing in each target:
 * Select `Add User-Defined Setting`
 * Add `CODE_SIGNING_REQUIRED` setting with a value of `NO`
 
+### Swift Package Manager
+* `open Package.swift` or double click `Package.swift` in Finder.
+* Xcode will open the project
+  * Choose a scheme for a library to build or test suite to run
+  * Choose a target platform by selecting the run destination along with the scheme
+
 ### Adding a New Firebase Pod
 
 See [AddNewPod.md](AddNewPod.md).