doc: update the Go faq about versioning

[flibustenet]

At the moment the Go faq says :

https://golang.org/doc/faq#get_version

Work is underway on an experimental package management tool, dep, to learn more about how tooling can help package management. More information can be found in the dep FAQ.

I believe it should be updated to mention vgo

[gopherbot]

Change https://golang.org/cl/114417 mentions this issue: doc: update version FAQ to mention the vgo initiative.

[gopherbot]

Change https://golang.org/cl/122408 mentions this issue: doc: describe modules in Go 1.11 release notes and Go FAQ

[gopherbot]

Change https://golang.org/cl/126777 mentions this issue: doc: add link to go command modules docs from Go 1.11 release notes