guix: README: Add darwin HOSTS entry

Author: dongcarl

contrib/guix/README.md

@@ -105,7 +105,8 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
 
   Override the space-separated list of platform triples for which to perform a
   bootstrappable build. _(defaults to "x86\_64-linux-gnu arm-linux-gnueabihf
-  aarch64-linux-gnu riscv64-linux-gnu x86_64-w64-mingw32")_
+  aarch64-linux-gnu riscv64-linux-gnu x86_64-w64-mingw32
+  x86_64-apple-darwin18")_
 
 * _**SOURCES_PATH**_