doc: update macOS information in README
doc: update macOS information in README

PR-URL: https://github.com/nodejs/node-gyp/pull/1323
Fixes: https://github.com/nodejs/node-gyp/issues/1295
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <c0a850ada395a21aea35ea96f1fadcdd1d6eb48b@gmail.com>
refack/node-gyp,TooTallNate/node-gyp,refack/node-gyp,refack/node-gyp,TooTallNate/node-gyp,TooTallNate/node-gyp,TooTallNate/node-gyp,refack/node-gyp,refack/node-gyp,TooTallNate/node-gyp