Add include tags (#26)

Author: kroikie

config/src/main/java/com/google/firebase/samples/config/Configure.java

@@ -38,13 +38,15 @@ public class Configure {
    * @return Access token.
    * @throws IOException
    */
+  // [START retrieve_access_token]
   private static String getAccessToken() throws IOException {
     GoogleCredential googleCredential = GoogleCredential
         .fromStream(new FileInputStream("service-account.json"))
         .createScoped(Arrays.asList(SCOPES));
     googleCredential.refreshToken();
     return googleCredential.getAccessToken();
   }
+  // [END retrieve_access_token]
 
   /**
    * Get current Firebase Remote Config template from server and store it locally.