doc: suggest xcode-select --install
If you don't have the full Xcode installed, the command-line tools are all you need.
Fixes: https://github.com/nodejs/node/issues/6449 Refs: https://github.com/nodejs/node/pull/9215
cc/ @nodejs/platform-macos
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc, build