Added initial badge (builtwith.svg/png) based on http://shields.io/ generated one

The initial badge is from the one generated by http://shields.io/ as [![built with gulp](http://img.shields.io/badge/built%20with-gulp-red.svg)](http://gulpjs.com) ... so now we can use these morkdown markups (once merged, please adjust to gulp repo): 
- for **PNG** [![built with gulp](https://raw.github.com/cyparu/artwork/master/builtwith.png)](http://gulpjs.com) use this: `[![built with gulp](https://raw.github.com/cyparu/artwork/master/builtwith.png)](http://gulpjs.com)`
- for **SVG** ??? *to be put & served from gulp.js main site ?!* - the https://raw.github.com/cyparu/artwork/master/builtwith.svg doesn't work...

**NOTE:** In context of gulp.js **PR** https://github.com/gulpjs/gulp/pull/405 and **issue**  https://github.com/gulpjs/gulp/issues/403