GitLab

Also insert MIGRATION.md into CHANGES.md.

Change-Id: I7eed691676d60db5493270b3cb559bb0e14de50c
Reviewed-on: https://code-review.googlesource.com/30310

Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>