Add example warning.

Author: abeisgoat

examples/SFVehicles/README.md

@@ -1,5 +1,7 @@
 ## GeoFire - SF Vehicles Example
 
+**Please note: This example will not function as the Open Data set it depends on has been deprecated. It is left here for historical purposes.**
+
 ### Setup
 
 The SF Vehicles example is based on Gradle. You can use Android Studio to import
@@ -19,4 +21,4 @@ SF Vehicles loads realtime transit data from a
 
 This sample loads its Firebase and GeoFire dependencies from Maven Central.
 If you modify and build GeoFire locally make sure to update to modify the gradle file to load GeoFire
-either directly or from your local maven repo.
+either directly or from your local maven repo.