docs: Update README.md

README.md

@@ -37,10 +37,17 @@ Yeah, simple as that.
 Maybe this library can be updated and more compatibility functions be added
 along the time, but there's no guarantee of that for now.
 
-``sys/tree.h`` was copied verbatim from the NetBSD source-tree by the Void Linux
-project, so it's covered by the BSD 2-Clause licence too.  
+``sys/tree.h`` was copied verbatim from the [NetBSD](https://netbsd.org)
+source-tree by the [Void Linux project](https://voidlinux.org), so it's covered
+by the BSD 2-Clause licence too;  
 ``sys/queue.h`` was also copied from NetBSD, but it's covered by the BSD 3-Clause
-licence and its ownership is claimed by The Regents of the University of California.
+licence, with its ownership claimed by The Regents of the University of California;  
+``sys/cdefs.h`` came from [Alpine Linux](https://alpinelinux.org) at some point
+in history, but it hasn't a copyright header --- and, to be honest, it isn't
+in fact copyrightable, since it does only provide some aliases to macros
+[utilized improperly](https://wiki.musl-libc.org/faq.html#Q:-When-compiling-something-against-musl,-I-get-error-messages-about-%3Ccode%3Esys/cdefs.h%3C/code%3E)
+by some folks in programs, since it should be only used internally at the
+GNU C Library.
 
 The test of ``error_at_line``(3) was taken from GNU m4 1.4.18 configure
 Shell script, so it's covered --- or better, not covered --- by the