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>
Merge pull request #601 from vogxn/master

Update README.md to include updatedeps target