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>

README.md

@@ -93,3 +93,6 @@ This will run tests for Packer core along with all the core builders and command
 
 If you make any changes to the code, run `make format` in order to automatically
 format the code according to Go standards.
+
+When new dependencies are added to packer you can use `make updatedeps` to
+get the latest and subsequently use `make` to compile and generate the `packer` binary.