Add note in BUILD about commonmark version

Change-Id: I2b99699b24c7f9eb4f1b070b2900c111916f31b2
diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index af15a27..92497c7 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -18,6 +18,7 @@
     repository = GERRIT,
   )
 
+  # commonmark must match the version used in Gitiles
   maven_jar(
     name = 'commonmark',
     artifact = 'com.atlassian.commonmark:commonmark:' + COMMONMARK_VERSION,