CONTRIBUTING.md: document codereview aliases

Fixes #636

Change-Id: I9bd9b3b3989c7cde501927d01c00ffbefd00caa0
Reviewed-on: https://code-review.googlesource.com/13214
Reviewed-by: Chris Broadfoot <cbro@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 2b4bac9..aed05dc 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -4,6 +4,12 @@
 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: