An example for Rate limit (#566)

* add rate limit example

* better comment

* add newline at end of file

examples/RateLimit.js

@@ -0,0 +1,69 @@
+/* RateLimit example */
+
+const uWS = require('../dist/uws.js');
+const port = 9001;
+
+const RateLimit = (limit, interval) => { let now = 0; const last = Symbol(), count = Symbol(); setInterval(() => ++now, interval); return ws => { if (ws[last] != now) { ws[last] = now; ws[count] = 1 } else { return ++ws[count] > limit } } }
+const rateLimit = RateLimit(1, 10000) //  limit is: 1 message per 10 seconds.
+const rateLimit2 = RateLimit(10, 2000) // limit per interval (milliseconds)
+
+
+const app = uWS./*SSL*/App({
+    key_file_name: 'misc/key.pem',
+    cert_file_name: 'misc/cert.pem',
+    passphrase: '1234'
+}).ws('/*', {
+    /* Options */
+    compression: uWS.SHARED_COMPRESSOR,
+    maxPayloadLength: 16 * 1024 * 1024,
+    idleTimeout: 10,
+    /* Handlers */
+    open: (ws) => {
+        console.log('A WebSocket connected!');
+    },
+    message: async (ws, message, isBinary) => {
+
+        // rateLimit(ws) returns true if over limit
+        if (rateLimit(ws)) {
+            /*
+                Do as you wish, personally, I prefer to inform the client, do Log,
+                 and close the connection if over limit happened.
+                or you can just ignore its message and just return. 
+            */
+            const dataToSendToClient = {
+                action: 'info',
+                data: {
+                    message: {
+                        english: 'over limit! please slow down.',
+                        farsi: 'یواش‌تر'
+                    }
+                }
+            }
+            await ws.send(JSON.stringify(dataToSendToClient));
+            console.warn('rate limit hit!');
+            return ws.end();
+        }
+
+        //or maybe different type of message or /endpoint needs different limit
+        if (ws.type2 && rateLimit2(ws)) {
+            console.warn('type2 is over limit');
+            return ws.end();
+        }
+
+        // rest of message code goes here ...
+    },
+    drain: (ws) => {
+        console.log('WebSocket backpressure: ' + ws.getBufferedAmount());
+    },
+    close: (ws, code, message) => {
+        console.log('WebSocket closed');
+    }
+}).any('/*', (res, req) => {
+    res.end('Nothing to see here!');
+}).listen(port, (token) => {
+    if (token) {
+        console.log('Listening to port ' + port);
+    } else {
+        console.log('Failed to listen to port ' + port);
+    }
+});