Documentation: Include Information on dep in Main README

[danielhelfand]

Currently, the main README for azure-sdk-for-go-samples includes instructions on running tests that includes running dep ensure. I think it would be helpful to link the section of the azure-sdk-for-go README that details Package Updates with the instruction for running this command. This will be helpful for any new Go developers who may be unfamiliar with dep.

Considerations:

• I personally found it easier to use go get -u github.com/Azure/azure-sdk-for-go for installing dependencies than using dep. Seeing as this is a sample repo, it may be better to recommend using go get -u github.com/Azure/azure-sdk-for-go rather than dep so that users can get going quicker with actually running the samples.

[danielhelfand]

@vladbarosan When you or someone has a moment, could you review this? Thanks!