Add steps to configure different environments for the test app. (#10491)

Author: visumickey

FirebaseSessions/README.md

@@ -9,6 +9,20 @@ Follow the [Main Firebase Readme](https://github.com/firebase/firebase-ios-sdk#d
  - `generate_project.sh` uses [cocoapods-generate](https://github.com/square/cocoapods-generate) to create an Xcode Workspace that has the SDK installed for all the SDK's supported platforms. This is useful for test-based development.
  - `generate_testapp.sh` generates and opens a test app with the Sessions SDK included. This is useful for developing the Sessions SDK against a real app.
 
+### Switching dev environments - Autopush/Staging/Prod
+
+SDK is configured to send events to different environments. To enforce different environments for sending events, we use an environment variable to configure the specific environment. Since environment variables are enforced in the context of the App, use the TestApp to send events to different environments after using the following configuration steps.
+
+- Enter "Edit scheme" - On the title bar menu "Product" > "Scheme" > "Edit Scheme"
+- Ensure "Run" is selected on the left tab
+- On the right hand side, choose the "Arguments" tab
+- Under the "Environment Variables", add the following variable to configure the environment
+   - For "AUTOPUSH" - "FirebaseSessionsRunEnvironment" -> "AUTOPUSH"/"autopush"
+   - For "STAGING" - "FirebaseSessionsRunEnvironment" -> "STAGING"/"staging"
+   - For "PROD" - "FirebaseSessionsRunEnvironment" -> "PROD"/"prod"
+
+NOTE: Default is PROD. Not configuring any flags would mean the events are sent to PROD environment.
+
 ### Debugging
 
 ### Command Line Arguments