[opnsense][FEATURE] add privileges to readme

[phoropter]

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.
• This is about a community plugin, not about Noctalia itself (those belong in the noctalia repo).

Plugin

opnsense

Feature type

Documentation improvement

Feature summary

Enumerate opnsense privileges needed for each tab of the plugin

Motivation / use case

To reduce how much guessing someone has to do when setting up api permissions in opnsense

Proposed solution

Could you please enumerate all the privileges needed for an opnsense user to have information populated for any given tab in the plugin?

[github-actions]

CC @davemhammer