Include reference to Prometheus Operator docs for user journey

[xenophenes]

Description

Currently, reading through solely the README for this project leaves users wondering how they can access services. It's a great resource for beginners, so the documentation should clearly denote this as well - on both the official docs and the GitHub README.

Type of change

What type of changes does your code introduce to the kube-prometheus? Put an x in the box that apply.

• [ ] CHANGE (fix or feature that would cause existing functionality to not work as expected)
• [ ] FEATURE (non-breaking change which adds functionality)
• [ ] BUGFIX (non-breaking change which fixes an issue)
• [x] ENHANCEMENT (non-breaking change which improves existing functionality)
• [ ] NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Changelog entry

Please put a one-line changelog entry below. Later this will be copied to the changelog file.

- Include reference to Prometheus Operator docs for user journey