Added simple built with gulp.js badge using shields.io badge service

[cyparu]

I added into README.md header this badge:
This should partially sort the #403

[phated]

Isn't red usually bad? In travis badges it means failing

[yocontra]

[cyparu]

Since gulp.js logo is hmm ... quite RED until a real (png/svg) badge will be created we should stick to it as well. As you can see, Grunt badge is orange+brown : .

Maybe a better idea will be to create a built-with-gulp.png/svg under https://github.com/gulpjs/artwork/ project that cand be used directly then. I'm not a designer, but we can use as defaults the the ones generated by shields.io http://img.shields.io/badge/built%20with-gulp.js-red.svg & http://img.shields.io/badge/built%20with-gulp.js-red.png

[yocontra]

@cyparu Yeah somebody should just make an SVG for it and we can host it on GH

[cyparu]

Additional context for github/badges: h5bp/lazyweb-requests#150 & https://github.com/badges/stability-badges

[cyparu]

It seams that for security reasons the SVG files can't be served directly from a github repository but should work from a gh-pages served (as gulpjs.com is).
Details here: https://groups.google.com/forum/#!topic/github/mS6Ea-sDwvs

As a result, the builtwith.svg (and for consistency builtwith.png) should be hosted on https://github.com/gulpjs/gulpjs.github.io (=> http://gulpjs.com) repo under img folder.

[yocontra]

Closing this since we will be doing a custom one