Document adding -fvisibility=hidden flag for macOS users

* -fvisibility=hidden flag for macOS user

Added section to remember the macOS user to add the `-fvisibility=hidden` flag.

PR-URL: nodejs/node-addon-api#460
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: John French

doc/setup.md

@@ -55,8 +55,19 @@ To use **N-API** in a native module:
 ```gyp
   'defines': [ 'NAPI_DISABLE_CPP_EXCEPTIONS' ],
 ```
-
-  4. Include `napi.h` in the native module code.
+  4. If you would like your native addon to support OSX, please also add the
+  following settings in the `binding.gyp` file:
+
+  ```gyp
+  ['OS=="mac"', {
+      'cflags+': ['-fvisibility=hidden'],
+      'xcode_settings': {
+        'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES', # -fvisibility=hidden
+      }
+  }]
+  ```
+
+  5. Include `napi.h` in the native module code.
      To ensure only ABI-stable APIs are used, DO NOT include
      `node.h`, `nan.h`, or `v8.h`.