Add note in WORKSPACE about updating BouncyCastle

When BC is updated in core Gerrit, it should also be updated
in bazlets, so that plugins get the same version.

Change-Id: Ibdc24214ba80ea5d215318efe736d7db0a259386
diff --git a/WORKSPACE b/WORKSPACE
index e261373..66729a9 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -612,6 +612,7 @@
     sha1 = "cd9886f498ee2ab2d994f0c779e5553b2c450416",
 )
 
+# When updading Bouncy Castle, also update it in bazlets.
 BC_VERS = "1.57"
 
 maven_jar(