Placeholder for performance improvement work items in roadmap.md

This PR asks for people to contribute and discuss ideas for improving Kubernetes performance in this shared Google Document: [Kubernetes Performance](https://docs.google.com/document/d/1-Oefi_zQCNAAysZpSkz4V4H_uQIGf90ZMj79Qevy4zE/edit#)

Once we have enough material the content of the document will be transplanted into a performance section of the roadmap document.