Add documentation around CustomRulePath in Settings file to README.md

[johlju]

PR Summary

• Add documentation around CustomRulePath in Settings file to README.md

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

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This change is 

[johlju]

@bergmeister as per request the updated docs.

[johlju]

Not sure why the build is failing. Looks like pipeline is broken?

[bergmeister]

Not sure why the build is failing. Looks like pipeline is broken?

It's a sporadic failure, for which I have PR #1632 to fix. I restarted the build, which worked this time.

[bergmeister]

Thanks for taking the time to write down your findings. Just one small suggestion please.