Update Function: Constraints for Kubernetes

[alexellis]

Update Function: Constraints for Kubernetes

Proposed change

The current documentation page yaml.md contains instructions for OS constraints with Docker Swarm, which is no longer supported.

This needs to be removed, but ideally we should have the equivalent for Kubernetes, to match operating system or CPU architecture.

Context

Potentially confusing for new users.

[alexellis]

/add label: help wanted, good first issue