Build instructions should mention govendor idiosynchrocies

[Skrylar]

Doing a straight checkout + clone results in an error from govendor. It claims "ketchup" is not a package or not in gopath. This can be worked around by:

• making a new directory
• setting GOPATH to that directory
• checking out to specifically src/github.com/ketchuphq/ketchup

The instructions currently read as though you can checkout anywhere (like many projects) and just run the makefile, but doing so produces a rather useless (UX-wise) error.