docs: document pod/container attribution and export flags

[domcyrus]

README follow-up for the pod-aware monitoring work (#10) and rustnet's kubernetes feature (domcyrus/rustnet#299):

• Features bullet for pod/container attribution with a link to the RustNet usage guide
• RustNet flags table: add --kubernetes MODE and --pcapng-export FILE, note that the JSON log carries the attribution
• Note that export files live inside the ephemeral pod and need kubectl cp before exit
• How It Works: mention the read-only /var/log mount used for pod/container name resolution
• Caveat on pinned image tags: attribution needs a tag newer than v1.4.0 or latest