Add note about aligning upgrades of prettify with plugins/gitiles

Change-Id: Id17160238316169c4bc50a1f2b2326e0a494b9c3

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index efd0e34..20f7379 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -58,6 +58,7 @@
     sha1 = "5871fb60dc68d67da54a663c3fd636a10a532948",
 )
 
+# When upgrading prettify it should also be updated in plugins/gitiles
 maven_jar(
     name = "prettify",
     artifact = "com.github.twalcari:java-prettify:1.2.2",