RFC documents for community feedback on proposed changes and improvements

See https://github.com/PowerShell/powershell-rfc/blob/master/RFC0000-RFC-Process.md for process information

Along with making PowerShell Open Source, we are also inviting the community to author RFCs on proposed design changes (instead of having long threads in issues). The PowerShell Team will meet once a week (depending on amount of feedback) to review the feedback and respond.

We'll continue to refine this process as we learn from it.