Add documentation for TLS configuration of NLK

[ciroque]

Proposed changes

Provide documentation for configuring NLK to use TLS for communications with NGINX Plus.

This is just the documentation, the implementation will come along later.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[4141done]

A few small notes but I suspect some things will change with implementation. Further improvements can come from user feedback. Might want to consider not merging this until implementation is complete but you've also provided ample warning so up to you.

[ciroque]

Decided it would be worse to throw this in with the implementation PR. Happy hunting.