Document caveats with running external script editors on macOS (don't run the .app, run the binary)

**Godot version:**
3.2.3.stable.official

**OS/device including version:**
OSX 10.14

**Issue description:**
Any time that I want to open a script it will open another copy of sublime text 3. This can get annoying if I wanted to edit multiple files.

**Steps to reproduce:**
My editor settings are:
Use External Editor: True.
Exec Path /Applications/Sublime Text.app/Contents/MacOS/Sublime Text
Exec flags: {project}

I also did use the recommended exec flags from the site: {file}:{line}.

Is it just that I selected the wrong file for the editor?