Add release.sh

This script can be used to build the Go part of SalsaFlow and upload it
to the specified GitHub release. It must be invoked from the top level
directory and it is expecting the repo to be cloned inside of a Go
workspace.
config: Use salsaflow.yml file name

Instead of gitflow.yml that was being used up until now.

Signed-off-by: Ondřej Kupka <ondra.cap@gmail.com>
Rename gitflow.yml to salsaflow.yml
release.sh: Create GitHub release drafts