godoc url in nut page.

[googollee]

It's convenience to add the godoc url in nut's detail page. The url should be easy converted from source url, which include in nuts.json.

[AlekSi]

Documentation at http://godoc.org/github.com/googollee/go-rest uses latest source code from GitHub. Link at http://gonuts.io/googollee/rest should point to documentation for latest published nut. In general, those two are different documentations.
I would like to add support for gonuts.io to godoc.org, but Gary Burd don't want to add gonuts.io-specific features at this time. Feel free to ask him.

[AlekSi]

Tracked as AlekSi/gonuts.io#17.