Add headers/README explaining use of directory

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

headers/README.md

@@ -0,0 +1,20 @@
+# NOTICE
+
+This directory contains include header files needed to compile BPF programs.
+
+The files are either copied from the kernel source (in subdir [linux/](linux))
+or "shadow" files that contain useful defines that are often used in kernel
+headers.
+
+For example [bpf/compiler.h](bpf/compiler.h) contains practical compile macros
+like `READ_ONCE` and `WRITE_ONCE` with verifier workarounds via
+`bpf_barrier()`.  And the `likely()` + `unlikely()` annotations.
+
+The include file [linux/bpf.h](linux/bpf.h) is the more central file that all
+BPF (kernel-side) programs include.  It is maintained in this directory,
+because this project knows what BPF features it uses, which makes the update
+cycle tied to the project itself.  We prefer not to depend on the OS distro
+kernel headers version of this file. (Hint, due to the use of `enum` instead of
+`define` the usual macro C-preprocessor define detection will not work. This is
+done on purpose to discourage userspace from detecting features via header file
+defines.).