link to license of musl-libc

Author: tkelman

LICENSE.md

@@ -42,6 +42,7 @@ External libraries, if used, include their own licenses:
 - [LIBUV](https://github.com/joyent/libuv/blob/master/LICENSE)
 - [LLVM](http://llvm.org/releases/3.0/LICENSE.TXT)
 - [MPFR](http://www.mpfr.org/mpfr-current/mpfr.html#Copying)
+- [MUSL](http://git.musl-libc.org/cgit/musl/tree/COPYRIGHT)
 - [OPENBLAS](https://raw.github.com/xianyi/OpenBLAS/master/LICENSE)
 - [PATCHELF](http://hydra.nixos.org/build/1524660/download/1/README)
 - [PCRE](http://www.pcre.org/licence.txt)