message: guix: README: Add darwin HOSTS entry
diff: @@ -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**_
  