Add gitgraph logo into README - close nicoespeon#11

Author: fabien0102

README.md

@@ -1,4 +1,6 @@
-gitgraph.js [![Build Status](https://secure.travis-ci.org/nicoespeon/gitgraph.js.png)](http://travis-ci.org/nicoespeon/gitgraph.js) [![devDependency Status](https://david-dm.org/nicoespeon/gitgraph.js/dev-status.png)](https://david-dm.org/nicoespeon/gitgraph.js#info=devDependencies)
+![gitgraph.js](/assets/logo/gitgraph-logo.png) 
+
+[![Build Status](https://secure.travis-ci.org/nicoespeon/gitgraph.js.png)](http://travis-ci.org/nicoespeon/gitgraph.js) [![devDependency Status](https://david-dm.org/nicoespeon/gitgraph.js/dev-status.png)](https://david-dm.org/nicoespeon/gitgraph.js#info=devDependencies)
 ===========
 
 A JavaScript library to draw pretty git graphs in the browser.