demo: document kernel rcvbuf sysctls for 64 MiB parser buffer

[armcconnell]

Summary

The parsers raise SO_RCVBUF to 64 MiB (#21) to absorb feed bursts, but the kernel clamps that request to net.core.rmem_max — which defaults far lower on a fresh host. Without raising it, the rcvbuf bump is only partially honored and sustained bursts can still drop at the socket layer.

This ports the operator sysctl guidance from the now-closed #13 into the demo README so the 64 MiB buffer can actually grow:

net.core.rmem_max=134217728
net.core.rmem_default=16777216
net.core.netdev_max_backlog=50000

Docs-only; no code change.

Context

• parser: decouple unix sink from UDP read path (both feeds) + honest datagram-loss metric #21 (merged) added the parser back-pressure fix + 64 MiB rcvbuf for both feeds.
• depthofbook-parser: prevent kernel UDP loss under bot back-pressure #13 (closed, superseded by parser: decouple unix sink from UDP read path (both feeds) + honest datagram-loss metric #21) carried this sysctl guidance only in its PR description; this preserves it.