Update README.md to include updatedeps target

When new dependencies are included builds on your localrepo will fail
until you have all the latest dependencies in the GOPATH. After doing
updatedeps, make will generate the packer binary.
Signed-off-by: Prasanna Santhanam <tsp@apache.org>

builder/qemu: Qemu builder

Conflicts:
	builder/amazon/common/run_config.go

Conflicts:
	post-processor/vagrant/post-processor.go

builder/digitalocean: add a droplet_name configuration value

Practically, this lets you set the hostname of the droplet
used for provisioning.