Docs: add very simple architecture diagram

Change-Id: Ib0552cafd80c8a602c984cf6ca9f7f4ca8e687fe
diff --git a/bindep.txt b/bindep.txt
index 8dffd0f..85254b4 100644
--- a/bindep.txt
+++ b/bindep.txt
@@ -8,6 +8,7 @@
 zookeeperd [platform:dpkg]
 build-essential [platform:dpkg]
 gcc [platform:rpm]
+graphviz [test]
 libssl-dev [platform:dpkg]
 openssl-devel [platform:rpm]
 libffi-dev [platform:dpkg]