Right now, this is what we have in CONTRIBUTING.md:

Contributing to Documentation

Contributing to documentation related to PowerShell

Please see the Contributor Guide in PowerShell/PowerShell-Docs.

Contributing to documentation related to maintaining or contributing to the PowerShell project

• When writing Markdown documentation, use [semantic linefeeds][].
  In most cases, it means "once clause / idea per line".
• Otherwise, these issues should be treated like any other issue in this repo.

I think we should have just one section "Contributing to Documentation" that will contain a link to Contributor Guide in PowerShell/PowerShell-Docs and nothing else. There is no need to have some additional rules for PowerShell project.

If something really needs to be specific for PowerShell project documentation that could be also added to Contributor Guide in PowerShell/PowerShell-Docs when we go public.