.gitignore: add

Add .gitignore for files generated when running the gddo-server.

Change-Id: Ifaef00ef42f3fb258acf747f02eb2d7787bce941
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274695
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>