docs(README.md): missing documentation on admins-ns

[josephmcasey]

Is your feature request related to a problem? Please describe.

When recently looking at unexpected functionality gaps with the label macros, I stumbled upon an old issue that pointed out the inability to make admin specific configurations with a resolution changeset to provide that functionality.

Describe the solution you'd like

Due to the cm needing to exactly match the key to a magic string value of admin-ns, I think the functionality would benefit from a explanation in the README.md.

Describe alternatives you've considered

Given the migration from cm to crd, I considered that this was intentionally left out of the documentation, so I could think a deprecation of complete removal of its reference from the README.md would be an alternative approach here.

Additional context

Referenced in README.md

kube-fluentd-operator/README.md

Lines 436 to 455 in f6ff431

	```xml
	admin-ns.conf:
	<match systemd.** docker kube.kube-system.** k8s.**>
	@type loggly
	loggly_url https://logs-01.loggly.com/inputs/TOKEN/tag/fluentd
	</match>
	<plugin test>
	@type s3
	aws_key_id YOUR_AWS_KEY_ID
	aws_sec_key YOUR_AWS_SECRET_KEY
	s3_bucket YOUR_S3_BUCKET_NAME
	s3_region AWS_REGION
	</plugin>
	<plugin staging>
	@type loggly
	loggly_url https://logs-01.loggly.com/inputs/TOKEN/tag/fluentd
	</plugin>
	```

Logic for usage

kube-fluentd-operator/config-reloader/generator/generator.go

Line 185 in 9958c98

err = util.WriteStringToFile(filepath.Join(outputDir, "admin-ns.conf"), renderedConfig)

[josephmcasey]

One question I had for the maintainers of this repository. Is it is desired that this functionality is compatible for the recommened FluentdConfig resources?