mirror: clarify Linux-only build support in Dependencies section

Signed-off-by: zhumengzhu <mengzhu.loveyou@gmail.com>

Author: zhumengzhu

README.md

@@ -54,6 +54,11 @@ reported on the mailing list nonetheless.
 Dependencies
 ------------
 
+> [!NOTE]
+> Building bpftool is only supported on Linux. The build process relies on
+> Linux-specific headers and tools, and is not expected to work on macOS or
+> other non-Linux systems.
+
 Required:
 
 - libelf