Add README files (#4857)

* Fix test app and add README's

* Undo app changes

* Undo app changes

* Add description and formatting to README file

Author: mrober

firebase-sessions/README.md

@@ -0,0 +1,27 @@
+# Firebase Sessions SDK
+
+The Firebase Sessions SDK is used by Crashlytics and Performance internally to measure sessions.
+
+## Building
+
+All Gradle commands should be run from the root of this repository.
+
+`./gradlew -PprojectsToPublish=":firebase-sessions" publishProjectsToMavenLocal`
+
+## Running Tests
+
+Unit tests:
+
+`./gradlew :firebase-sessions:test`
+
+Integration tests, requiring a running and connected device (emulator or real):
+
+`./gradlew :firebase-sessions:connectedAndroidTest`
+
+## Code Formatting
+
+Format Kotlin code in this SDK in Android Studio using
+the [ktfmt plugin](https://plugins.jetbrains.com/plugin/14912-ktfmt) with code style to
+**Google (internal)**, or by running:
+
+`./gradlew :firebase-sessions:ktfmtFormat`

firebase-sessions/test-app/README.md

@@ -0,0 +1,17 @@
+# Firebase Sessions Test App
+
+## Setup
+
+Download the `google-services.json` file
+from [Firebase Console](https://console.firebase.google.com/) (for whatever Firebase project you
+have or want to integrate the `test-app`) and store it under the current directory.
+
+Note: The [Package name](https://firebase.google.com/docs/android/setup#register-app) for your app
+created on the Firebase Console (for which the `google-services.json` is downloaded) must match
+the [applicationId](https://developer.android.com/studio/build/application-id.html) declared in
+the `test-app/test-app.gradle.kts` for the app to link to Firebase.
+
+## Running
+
+Run the test app directly from Android Studio by selecting and running
+the `firebase-sessions.test-app` run configuration.