[DOC] Info about cmd client

[XavierVal]

Under https://github.com/telefonicaid/iotagent-mqtt#-command-line-client please add that is needed a config file called "client-config.js" to use de CMD client with containing the basic config needed:

var config = {};

config.mqtt = {
    host: 'localhost',
    port: 1883,
    options: {
        keepalive: 0,
        connectTimeout: 60 * 60 * 1000
    }
};

config.device = {
    id: 'myDeviceId',
    apikey: '1234'
}

module.exports = config;