Add plugins info in usage example

[magicznyleszek]

Usage example is missing info about plugin :)

// karma.conf.js 
module.exports = function(config) {
    // ... snip 
    frameworks: ['websocket-server'],
    websocketServer: {
        port: 8889,
        beforeStart: (server) => {
            server.on('request', (req) => {
                console.log(new Date() + ' new websocket request...');
            });
        },
        afterStart: (server) => {
            console.log('Server now listening!');
        }
    },
    plugins: ['karma-websocket-server'],
};