Merge pull request #828 from morfey13/patch-1

use `const` instead `var` in example from README.markdown

Author: Stuk

README.markdown

@@ -7,11 +7,11 @@ lovely and simple API.
 See https://stuk.github.io/jszip for all the documentation.
 
 ```javascript
-var zip = new JSZip();
+const zip = new JSZip();
 
 zip.file("Hello.txt", "Hello World\n");
 
-var img = zip.folder("images");
+const img = zip.folder("images");
 img.file("smile.gif", imgData, {base64: true});
 
 zip.generateAsync({type:"blob"}).then(function(content) {