CONTRIBUTING: adds description of amending changes
This change brought on by confusion of an external contributor,
the directions.
1. Make changes and create a change by running `git codereview change <name>`,
provide a commit message, and use `git codereview mail` to create a Gerrit CL.
-1. Keep amending to the change and mail as your receive feedback.
+1. Keep amending to the change with `git codereview change` and mail as your receive
+feedback. Each new mailed amendment will create a new patch set for your change in Gerrit.
## Integration Tests
