[grok] README Known Limitations omit OpenWrtPlugin

[sfloess]

[grok] Post-verification nits @ 0.9.

Problem

tftpos/plugins/openwrt.py ships and is registered:

openwrt = "tftpos.plugins.openwrt:OpenWrtPlugin"

README Known Limitations still says only:

Plugin ecosystem — tftp-os ships StaticFirmwarePlugin for simple {distro_root}/{os_family}/{os_version}/firmware.bin layouts…

That understates the built-in surface and will send app authors to write custom OpenWRT path logic unnecessarily.

Acceptance criteria

• Known Limitations (and plugin docs if needed) mention OpenWrtPlugin and its path convention (…/openwrt/{version}/targets/{target}/{subtarget}/{filename})
• Note that both static and openwrt still require register(..., distro_root=...) (discover does not auto-instantiate)
• Optional: one-line example registering OpenWrtPlugin in README or PLUGIN_GUIDE

Notes

Docs only.