docs: Add a little diagram of how I think about Tilde Friends.

docs/model.md

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+# Model
+
+A reasonable mental model of Tilde Friends is as a virtual computer.  User
+interace is through a web browser.  Communication with the outside world is
+through the Secure Scuttlebutt (SSB) network protocol.  Persistence is through
+an SSB store and an additional key-value store in an sqlite database.
+
+The schema for the sqlite database is primarily a `messages` table and a
+`blobs` table, which are what one would expect from the SSB specifications.
+
+```dot
+digraph {
+    web_browser -> tilde_friends_web_interface [dir=both];
+    web_browser [shape=rect,label="Web Browser"];
+    subgraph cluster_tilde_friends {
+        label = "Tilde Friends";
+        tilde_friends_web_interface -> example_app [dir=both];
+        subgraph cluster_sandbox {
+            label = "app sandbox";
+            example_app;
+        }
+        example_app -> tilde_friends_core;
+        tilde_friends_core -> example_app;
+        tilde_friends_web_interface -> tilde_friends_core;
+        tilde_friends_core -> "db.sqlite";
+        tilde_friends_core -> ssb;
+        "db.sqlite" [shape=cylinder];
+    }
+    ssb -> other_ssb_clients [label="Secure Handshake",dir=both];
+    other_ssb_clients [shape=rect,label="SSB Peers"];
+}
+```