CONTRIBUTING: adds description of amending changes
CONTRIBUTING: adds description of amending changes

This change brought on by confusion of an external contributor,
https://code-review.googlesource.com/#/c/10590/

Change-Id: I5ad877164d983918b23c464a6c48afa6ab1d200b
Reviewed-on: https://code-review.googlesource.com/10630
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go