Add vendoring workflow to README.md

[Will Rouesnel]

Currently there's no vendoring workflow noted in the README.md

At a best guess the gvt tool seems compatible, but gvm is mentioned in the commit logs.

Could this be clarified?

[Timothy]

From the travis.yml, it looks like the project is using go modules now

[Sergei Aleksandrovich]

Yup, I use Go modules for Hivemind now