[docs] Use new JavaScript syntax inside the README (#3360)

Author: EmmanuelDemey

Readme.md

@@ -55,10 +55,10 @@ Any serializable data structures can be emitted, including:
 Sample code:
 
 ```js
-io.on('connection', function(socket){
-  socket.emit('request', /* */); // emit an event to the socket
-  io.emit('broadcast', /* */); // emit an event to all connected sockets
-  socket.on('reply', function(){ /* */ }); // listen to the event
+io.on('connection', socket => {
+  socket.emit('request', /* … */); // emit an event to the socket
+  io.emit('broadcast', /* … */); // emit an event to all connected sockets
+  socket.on('reply', () => { /* … */ }); // listen to the event
 });
 ```
 
@@ -84,7 +84,7 @@ This is a useful feature to send notifications to a group of users, or to a give
 ## Installation
 
 ```bash
-npm install socket.io --save
+npm install socket.io
 ```
 
 ## How to use
@@ -93,20 +93,20 @@ The following example attaches socket.io to a plain Node.JS
 HTTP server listening on port `3000`.
 
 ```js
-var server = require('http').createServer();
-var io = require('socket.io')(server);
-io.on('connection', function(client){
-  client.on('event', function(data){});
-  client.on('disconnect', function(){});
+const server = require('http').createServer();
+const io = require('socket.io')(server);
+io.on('connection', client => {
+  client.on('event', data => { /* … */ });
+  client.on('disconnect', () => { /* … */ });
 });
 server.listen(3000);
 ```
 
 ### Standalone
 
 ```js
-var io = require('socket.io')();
-io.on('connection', function(client){});
+const io = require('socket.io')();
+io.on('connection', client => { ... });
 io.listen(3000);
 ```
 
@@ -118,10 +118,10 @@ to pass the `Server` to `socket.io`, and not the express application
 function. Also make sure to call `.listen` on the `server`, not the `app`.
 
 ```js
-var app = require('express')();
-var server = require('http').createServer(app);
-var io = require('socket.io')(server);
-io.on('connection', function(){ /* … */ });
+const app = require('express')();
+const server = require('http').createServer(app);
+const io = require('socket.io')(server);
+io.on('connection', () => { /* … */ });
 server.listen(3000);
 ```
 
@@ -131,10 +131,10 @@ Like Express.JS, Koa works by exposing an application as a request
 handler function, but only by calling the `callback` method.
 
 ```js
-var app = require('koa')();
-var server = require('http').createServer(app.callback());
-var io = require('socket.io')(server);
-io.on('connection', function(){ /* … */ });
+const app = require('koa')();
+const server = require('http').createServer(app.callback());
+const io = require('socket.io')(server);
+io.on('connection', () => { /* … */ });
 server.listen(3000);
 ```