Updates Readme.md for clarity

I'm new Godep and it took me a while to understand that `godep restore` is basically the opposite of `godep save` and was slightly alarmed that it would overwrite files in my $GOPATH. The source of my confusion was the Add/Update section of the readme which has restore as the first step. Furthermore, that section does not actually work for updating a package. Looks like the update command was added recently for this specific purpose, so I broke out the two sections to have their own explanation.