dep constraint example is wrong in the README

[srowles]

You have to constrain at the root package level

e.g.

[[constraint]]
  name = "github.com/IBM/vpc-go-sdk"
  version = "0.0.1"

I'd recommend suggesting people use:

dep ensure -add github.com/IBM/vpc-go-sdk/vpcv1

[srowles]

PR: #9

fixes the dep example, and also adjusts the example code to be more go idiomatic with inline struct creation, and demonstrates the use of the StringPtr method from the core sdk.