add: graphviz design doc (#65)

assets/design.dot

@@ -0,0 +1,24 @@
+digraph Kodiak {
+
+  "GitHub"[shape=doublecircle]
+
+  "GitHub" -> "HTTP Webhook"[label="HTTP events", weight=0]
+
+
+  subgraph cluster_kodiak {
+    label = "Kodiak"
+
+    "HTTP Webhook" -> Redis[label="write event to queue", weight=0]
+    Redis[shape=cylinder]
+
+    "Repo Queue Worker" -> Redis [label="pull off mergeable\n PRs and merge"]
+    "Repo Queue Worker" -> "GitHub"
+    "Async Worker" -> "GitHub"
+
+    "Async Worker" -> Redis[label="write any mergeable PRs\nto per repo queue"]
+
+    "Event Queue Worker" -> Redis[label="Left blocking pop \nfrom event queue"]
+    "Event Queue Worker" -> "Async Worker"[style="dashed", label="create async task \nfor each event"]
+  }
+
+}