doc: add macOS Catalina software update info
doc: add macOS Catalina software update info

PR-URL: https://github.com/nodejs/node-gyp/pull/2078
Reviewed-By: Christian Clauss <585dd401264c1e46d17ebec1862e5d2a9b6a6c5d@me.com>
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp