Add describe endpoint

[glynhudson]

To allow OpenEVSE to be discovered by https://github.com/emoncms/find

void handleDescribe(AsyncWebServerRequest *request) {
  AsyncWebServerResponse *response = request->beginResponse(200, "text/plain", "openevse");
  response->addHeader("Access-Control-Allow-Origin", "*");  
  request->send(response);
}

web_server_setup()
 {
server.on("/emoncms/describe", handleDescribe);
}

e.g. openenergymonitor/EmonESP@839379d?diff=unified

[glynhudson]