Add Kotlin format instructions to the README file (#4798)

Author: mrober

README.md

@@ -251,8 +251,10 @@ projects may be published as follows.
 
 ### Code Formatting
 
-Code in this repo is formatted with the google-java-format tool. You can enable
-this formatting in Android Studio by downloading and installing the
+#### Java
+
+Java code in this repo is formatted with the google-java-format tool. You can
+enable this formatting in Android Studio by downloading and installing the
 [google-java-format plugin](https://github.com/google/google-java-format).
 The plugin is disabled by default, but the repo contains configuration information
 and links to additional plugins.
@@ -262,6 +264,18 @@ To run formatting on your entire project you can run
 ./gradlew :<firebase-project>:googleJavaFormat
 ```
 
+#### Kotlin
+
+Kotlin code in this repo is formatted with the `ktfmt` tool. You can enable
+this formatting in Android Studio by downloading and installing the
+[ktfmt plugin](https://plugins.jetbrains.com/plugin/14912-ktfmt).
+Enable the plugin in Preferences | Editor | ktfmt Settings. and set code style to Google (internal).
+
+To run formatting on your entire project you can run
+```bash
+./gradlew :<firebase-project>:ktfmtFormat
+```
+
 ### Contributing
 
 We love contributions! Please read our