kubebuilder/quickstart: add instruction for enabling status subresource

[hasbro17]

Description of the change:
Add docs for enabling status subresource in the kubebuilder quickstart guide.

Motivation for the change:
Resolves the error I saw in #2827 (comment) where the controller failed to update the status subresource with the status writer client.
https://github.com/operator-framework/operator-sdk/blob/master/example/kb-memcached-operator/memcached_controller.go.tmpl#L114

[camilamacedo86]

/lgtm
/approve

[hasbro17]

@asmacdo Any idea why the netlify checks might have failed?
https://github.com/operator-framework/operator-sdk/pull/2897/checks?check_run_id=613752189
https://app.netlify.com/sites/operator-sdk/deploys/5ea22c115c492f0006ce0ec2

Building sites …
5:01:08 PM: fatal error:
5:01:08 PM: concurrent map read and map write
...

Not sure how to rerun them if this is a flake.

[camilamacedo86]

I saw this error occurs before. No idea too.

[estroz]

Nit: wording, comment is implied by marker.

Suggested change

	Add the `+kubebuilder:subresource:status` [marker comment][status_marker] to enable the [status subresource][status_subresource] for the CRD so that the controller can update the CR status without changing the rest of the CR object:
	Add the `+kubebuilder:subresource:status` [marker][status_marker] to add a [status subresource][status_subresource] to the CRD manifest so that the controller can update CR statuses efficiently:

[hasbro17]

Agree with the other nits but I'd still want to highlight exactly why we're enabling the status subresource rather than say efficiently. That behavior is not obvious and it's better to be verbose about it.
Part of the reason why I had this error in the first place.

[estroz]

One nit, otherwise

/lgtm

[bharathi-tenneti]

/lgtm

[openshift-ci-robot]

New changes are detected. LGTM label has been removed.