add github link in the official site

[xialvjun]

as the title

[camchenry]

@hildjj I believe this can be closed out? There are a few links to GitHub on the website.

1. In the footer
   
2. On the "Development" page
   

[hildjj]

I was thinking maybe we should make it a little more in-your-face, like a fork me on github ribbon in the upper right corner of every page.

[hildjj]

Anyone who is still annoyed by this, please re-open, or send a PR.