Add description and formatting to README file

Author: mrober

firebase-sessions/README.md

@@ -1,5 +1,7 @@
 # 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.
@@ -15,3 +17,11 @@ Unit tests:
 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`