all: add some README.md files describing the purpose of our sources

src/linux-headers/README.md

@@ -0,0 +1,10 @@
+linux-headers
+=============
+
+Contains a copy of Linux UAPI kernel headers.
+When we build against an older kernel, we may
+still want to unconditionally build against a
+certain version of kernel API.
+
+These headers should be taken without modification
+from Linux.