added installation for pathogen

There was a *todo* in place of the pathogen installation. I found this repo from "https://github.com/nsf/gocode/blob/9dba28b45d8e7a84961e780b44444d766e4b2225/README.md" who said said, "Alternatively take a look at the vundle/pathogen friendly repo: https://github.com/Blackrush/vim-gocode," so I figured it would be appropriate for the repo to have a pathogen install. Plus it's a really easy change (2 lines).