document basic configuration regarding prerequisites

[Happy86]

Proposed changes

After replying to my own discussion thread in #1261 (problem using a unix socket for the nginx stub_status) this PR should add the missing documentation.

Unfortunately at the moment there is no documentation on how to properly use --nginx.scrape-uri= if you want to connect to a socket for the stub_status.

I also added sample configuration.

Improvements and criticism for this are welcome. :-)

And yes: Instead of guessing right in the end I could have just read the source (line 68 in exporter.go).

Checklist

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

• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[nginx-bot]

Hi @Happy86! Welcome to the project! 🎉

Thanks for opening this pull request!
Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.

[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[Happy86]

I have hereby read the F5 CLA and agree to its terms