Added usage example in README.

Also made the DHT constructor a bit more user friendly.

README.md

@@ -4,3 +4,15 @@ An implementation of the Kademlia DHT.
 Supports:
  - BEP-42 with `sse4_crc32` optional dependency.
  - BEP-44 with `ed25519-supercop` optional dependency.
+
+## Usage
+```javascript
+	import DHT from 'kadem';
+
+	// load DHT from stored state file; if it doesn't
+	// exist or is not provided it'll bootstrap itself.
+	const dht = DHT.load('.dht_state');
+	dht.listen(/** listening port, can be anything */ 8468);
+
+	const value = await dht.get({ k: key, salt: salt });
+```

package.json

@@ -2,7 +2,7 @@
   "name": "kadem",
   "version": "1.0.0",
   "description": "",
-  "main": "index.js",
+  "main": "src/index.js",
   "scripts": {
     "test": "mocha test/**/*.js"
   },

src/index.js

@@ -155,6 +155,8 @@ util.inherits(DHT, EventEmitter);
  * @param {string} filename The filename to load from.
  */
 DHT.load = function(filename) {
+  if (filename == undefined || !fs.existsSync(filename)) return new DHT();
+
   let state = JSON.parse(fs.readFileSync(filename).toString('utf-8'));
   return new DHT({
     K: state.K,