Update readme, add test for no urls found

Author: s0ph1e

README.md

@@ -13,11 +13,13 @@ Parse urls from css file
 
 ## Usage
 ```javascript
-var parseCssUrls = require('css-url-parser'); 
+var parseCssUrls = require('css-url-parser');
 
 var css = '@import "a.css"; .image { background-image: url(images/img.png); }';
 var cssUrls = parseCssUrls(css);
 
 console.log(cssUrls);   // [ 'a.css', 'images/img.png' ]
 ```
 
+It ignores duplicated urls and base64 encoded resources.
+If no urls found empty array will be returned.

test/css-parser-test.js

@@ -50,13 +50,23 @@ var should = require('should')
 			';
 
 			var urls = parseCssUrls(text);
+			urls.should.be.instanceof(Array);
 			urls.should.be.empty;
 		});
 
 		it('should ignore empty urls', function(){
 			var text = 'div.image { background-image: url(""); } ';
 
 			var urls = parseCssUrls(text);
+			urls.should.be.instanceof(Array);
+			urls.should.be.empty;
+		});
+
+		it('should return empty array if no urls were found', function(){
+			var text = 'no css urls should be found in this text';
+
+			var urls = parseCssUrls(text);
+			urls.should.be.instanceof(Array);
 			urls.should.be.empty;
 		});
 	});