assets: SVG badge for Retina displays

A blue badge generated with http://shields.io/ using the same text as before, but on blue (`#5272B4`) instead of green. 

![GoDoc](http://img.shields.io/badge/godoc-reference-5272B4.svg)

Keeping the PNG for existing badges in the wild.

![GoDoc](http://img.shields.io/badge/godoc-reference-5272B4.png)