docs: add note about PodSecurity in Rook-Ceph guide

[zonorti]

Pull Request

What? (description)

A short, but helpful addition to rook-ceph guide. I stumbled upon this issue myself and hope it will help others.

Why? (reasoning)

Just following the guide didn't get me a running cluster and it took some time to figure out why it's failing.

Acceptance

Please use the following checklist:

• [ ] you linked an issue (if applicable)
• [ ] you included tests (if applicable)
• [ ] you ran conformance (make conformance)
• [ ] you formatted your code (make fmt)
• [ ] you linted your code (make lint)
• [ ] you generated documentation (make docs)
• [ ] you ran unit-tests (make unit-tests)

See make help for a description of the available targets.

Only docs changes, so I didn't run anything mentioned.

[smira]

/m

[zonorti]

Wow, that was fast, thanks!