README: Describe file transfer support (alpha).

Author: pfalcon

README.md

@@ -7,3 +7,20 @@ accessing a MicroPython REPL over WebSockets.
 To start WebREPL terminal client, clone or download this repository
 (in full) and open webrepl.html in a browser. Recent versions of
 Firefox and Chrome (or Chromium) are supported.
+
+WebREPL file transfer
+---------------------
+
+WebREPL protocol includes experimental support for file transfer.
+This feature is currently in alpha and has known issues on systems
+which have it enabled (ESP8266).
+
+To use WebREPL file transfer capabilities, a separate command line
+utility is provided, webrepl_cli.py (file transfer is not supported
+via webrepl.html client). Run
+
+    webrepl_cli.py --help
+
+to see usage information. Note that there can be only one active
+WebREPL connection, so while webrepl.html is connected to device,
+webrepl_cli.py can't transfer files, and vice versa.