Update README with kernel headers information

Include kernel headers >=4.1 as a prerequisite.

README.md

@@ -43,6 +43,7 @@ To get started using this toolchain, one needs:
   * `CONFIG_BPF_JIT=y`
   * `CONFIG_HAVE_BPF_JIT=y`
   * `CONFIG_BPF_EVENTS=y` [optional, for kprobes]
+* Linux kernel headers, 4.1 or newer
 * LLVM 3.7 or newer, compiled with BPF support (default=on)
 * Clang 3.7, built from the same tree as LLVM
 * pyroute2, version X.X (currently master, tag TBD) or newer