PERA-1348 :: Add in Polling Diagram

ARCHITECTURE.md

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+# Polling
+
+<img src="mermaid/polling.png" width="300">
+
+
+```sh
+flowchart TD
+subgraph ABC["**Polling**"]
+%% Nodes
+    A("Initialize Polling")
+    B("Get local account addresses and earliest fetch round")
+    C("Post this data to backend and wait for response")
+    D["Is there a change in any of those accounts since earliest fetch round?"]
+    E("Update account & ASA cache")
+    F("Wait 3.5 seconds")
+
+%% Edge connections between nodes
+    A --> B --> C --> D -- Yes --> E
+    D -- No --> F
+    E --> F
+    F --> B
+
+%% Node Shapes
+    D@{shape: diam}
+end
+```