Add config-features.yaml symlink to make `ko apply -f config/` work. (#8170)
Add config-features.yaml symlink to make `ko apply -f config/` work. (#8170)

Currently what we document in DEVELOPMENT.md is `ko apply -f config/`,
thus we should add the symlink to be consistent.

If we agree on https://github.com/knative/serving/pull/8082, we can
remove it later.
knative/serving,knative/serving