Add doc about Go migration feature flags
Related to discussion here: omnibus-gitlab!3215 (comment 161478270).
This MR is for updating the README to add some information how to enable feature flags related to Go migration.
It includes a list of features that can be enabled (based on https://gitlab.com/gitlab-org/gitlab-shell/blob/master/go/internal/command/commandargs/command_args.go#L11-14). I imagine the list will grow as we migrate more feature to Go.
Edited by Patrick Bajao