README.devices: document conn=bt/ extensions for more optional parameters

The conn=bt/ syntax grew support for optional additional parameters. Add
them to the README.devices page for awareness.

Author: gsigh

README.devices

@@ -201,14 +201,16 @@ Formal syntax for serial communication:
    path may contain slashes
    path and serno are "greedy" (span to the end of the spec)
  - Bluetooth Classic and Bluetooth Low Energy (BLE):
-   conn=bt/<conn>/<addr>
+   conn=bt/<conn>/<addr>[/param=value]
    conn can be: rfcomm, ble122, nrf51, cc254x
    addr can be "dense" or separated, bt/cc254x/0123456789ab or
      bt/rfcomm/11-22-33-44-55-66 or bt/ble122/88:6b:12:34:56:78
      (note that colons may not be available when the conn= spec is taken
      from a string that separates fields by colon, e.g. in the "--driver
      <name>:conn=<spec>" example, that is why the dense form and the use
      of dashes for separation are supported)
+   additional parameter keywords can be: channel, handle_rx, handle_tx,
+     handle_cccd, value_cccd
 
 Some of the drivers implement a default for the connection. Some of the
 drivers can auto-detect USB connected devices.