doc: macOS Catalina add two commands
doc: macOS Catalina add two commands

These steps have proven effective for some users.

PR-URL: https://github.com/nodejs/node-gyp/pull/1962
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

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