doc: document the dev socket

src/backend/doc/dev_socket.md

@@ -0,0 +1,15 @@
+## Backend - dev socket
+
+The "dev socket" allows you to interact with Puter's backend by running commands.
+It's a UNIX socket created in Puter's runtime directory
+(typically `./volatile/runtime`, or `/var/puter` for production instances).
+
+When in the runtime directory, you can connect to the socket with your tool
+of choice. For example, using `nc` as well as `rlwrap` to get readline history:
+
+```
+rlwrap nc -U ./dev.sock
+```
+
+If it is successful you will see a message with instructions. At this point
+you may enter a command. Enter the `help` command to see a list of commands.