Add link to GoDoc.org

[mewmew]

We should make sure to add links to GoDoc.org

• https://godoc.org/engo.io/ecs
• https://godoc.org/engo.io/engo
• https://godoc.org/engo.io/engo/core

Based on the discussion at https://gitter.im/EngoEngine/engo?at=57240e47e10a59c0610750d4

@otraore
I also think before v1 we need a site redesign

@mewmew
With or without a site redesign, it would be great if there was a link to GoDoc.org from engo.io. Perhaps there is one and I failed to find it?

@otraore
I don’t think there is but I do agree there should be one

[Noofbiz]

Fixed by #28