Explain why Gitblit plugin 2.13 is not built

Change-Id: Id13baf6c11bd1c52d167ab8760d4205c3810e68a
diff --git a/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml b/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
index b01a37d..eeb93f9 100644
--- a/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
+++ b/jenkins/gerrit-buck-bazel-plugin-gitblit.yaml
@@ -85,6 +85,7 @@
       - 'plugin-gitblit-{branch}':
           targets: plugins/gitblit
           branch:
+            # stable-2.13 missing because Gitblit Lucene-level isn't compatible with the Gerrit's version
             - stable-2.12
             - stable-2.11
             - stable-2.10