Update install.md to add instructions for macOS

[Alyetama]

Without coreutils, installation from source on macOS fails with error:

chmod +x "./voice2json/voice2json.sh"
install -D "./voice2json/voice2json.sh" "./voice2json/.venv/bin/voice2json"
install: illegal option -- D
usage: install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 file2
       install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 ... fileN directory
       install -d [-v] [-g group] [-m mode] [-o owner] directory ...
make: *** [install-voice2json] Error 64

After installing coreutils with Homebrew, I was able to install voice2json successfully on my macOS.

Reference: https://apple.stackexchange.com/a/201030

[synesthesiam]

Thanks!