Add module diagram

README.md

@@ -150,6 +150,35 @@ WireGuard can also pierce firewalls, but it requires root permissions,
 and the client must be a WireGuard node. PTTH allows any normal HTTP client
 such as curl, Firefox, etc.
 
+## Module overview
+
+```
++------------+       +-------------+       +------------------+
+| ptth_relay |       | ptth_server |       | ptth_file_server |
++------------+       +-------------+       +------------------+
+            |         |           |          |
+             \       /             \        /
+              V     V               V      V
+           +------------+       +-------------+
+           | http_serde |       | file_server |
+           +------------+       +-------------+
+```
+
+The top-level binaries are ptth_relay, ptth_server, and ptth_file_server.
+
+ptth_relay should run on a well-known public server, behind an HTTPS proxy
+such as Caddy, Nginx, or Apache.
+
+ptth_file_server is a standalone HTTP file server, similar to Python2's
+`-m SimpleHTTPServer` command, or Python3's `-m http.server`.
+
+ptth_server and ptth_file_server use the `file_server` module. ptth_server
+will connect out to a ptth_relay instance and serve files through the reverse
+HTTP tunnel.
+
+The http_serde module is shared by ptth_relay and ptth_server so that they
+can communicate with each other easily.
+
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