Add attach_xdp to reference_guide.md (#3450)
Add attach_xdp to reference_guide.md (#3450)

- Add attach_xdp to reference_guide.md
- Add description about flags
iovisor/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc