Move script parity comment into scripts README

bin/README.md

+# Cross-track consistency
+
+Many of these scripts are shared between the Java and Kotlin tracks. If you make an update to a script in one of these tracks, please also update the same script in the other track if appropriate. Thank you!

bin/journey-test.sh

 #!/usr/bin/env bash
 
-# This script is shared between the Java and Kotlin tracks.  If you make an update to this script on
-# one track, please create a companion PR to merge it in to the other.  Thank you!
 TRACK=java
 TRACK_REPO="$TRACK"
 TRACK_SRC_EXT="java"