readme: add telnet usage example gif

PROTOCOL.md

@@ -28,4 +28,8 @@ Sending `EOT` (`0x04`) closes connection.
 Each response is represented by one or more lines, each ending with `\r\n`, plus
 extra `\r\n` in the end.
 
-First line is always a status, which may be either `ok` or `err`.
+First line is always a status, which may be either `ok` or `err`.
+
+## Usage example
+
+![inverterd-telnet](inverterd-telnet.gif)

README.md

@@ -7,7 +7,7 @@ is planned.
 - `inverterctl` is a full-featured command line utility with all P18 commands
   supported.
 - `inverterd` is a daemon that starts a TCP server and implements simple text-based
-  telnet-compatible chat [protocol](PROTOCOL.md). It supports all commands and options
+  [telnet-compatible](inverterd-telnet.gif) chat [protocol](PROTOCOL.md). It supports all commands and options
   supported by inverterctl, and it's supposed to be used instead of interterctl in
   multi-user or multi-threaded scenarios, where there may be more than one simultaneous
   request to device, to avoid errors or device lockups.