CONVENTIONS.md: Update details on port-mappings

CONVENTIONS.md: Update details on port-mappings

After discussion with the CNI maintainers and representatives from Mesos and Kubernetes we've settled on this approach for passing the dynamic port mapping information from runtimes to plugins.

Including it in the plugin config allows "operators" to signal that they expect port mapping information to be passed to a specific plugin (or plugins). This follows the model that config passed in the plugin config should be acted upon by a plugin.

Runtimes can optionally return an error to users if they find no plugin in the chain that supports port mappings.

I'm sure we'd all like to see this merged soon so I'll try to turn any feedback quickly (and I'm happy to get any and all feedback - spellings, unclear working, wrong JSON terminology etc..)