docs: document OpenWrt cfg80211 backport builds

27af5ad added the CFG80211_VERSION make variable for OpenWrt builds that
use a newer mac80211 backport than the target kernel, but it is only
described in a Makefile comment. Document how to pass it from an OpenWrt
package Makefile, link the notes from the installation steps, and add
the build script and the kernel/backports combinations verified for
issue #94.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -67,6 +67,10 @@ has reached `a69c:8d80` but firmware startup still fails, see
 
 ### Installation Steps
 
+For OpenWrt SDK/buildroot builds, see [OpenWrt build notes](OPENWRT.md).
+OpenWrt can use a newer cfg80211 backport than its target kernel; these builds
+must pass the wireless stack version separately.
+
 #### Method 1: [Quick Installation](INSTALL_SCRIPT.md) (Recommended)
 
 #### Method 2: Manual Installation