CONTRIBUTING.md: document codereview aliases
Fixes
1. `go get golang.org/x/review/git-codereview` to install the code reviewing tool.
1. You will need to ensure that your `GOBIN` directory (by default
`$GOPATH/bin`) is in your `PATH` so that git can find the command.
+ 1. If you would like, you may want to set up aliases for git-codereview,
+ such that `git codereview change` becomes `git change`. See the
+ [godoc](https://godoc.org/golang.org/x/review/git-codereview) for details.
+ 1. Should you run into issues with the git-codereview tool, please note
+ that all error messages will assume that you have set up these
+ aliases.
1. Get the cloud package by running `go get -d cloud.google.com/go`.
1. If you have already checked out the source, make sure that the remote git
origin is https://code.googlesource.com/gocloud:
