Update 02-install-openfl.html.md

* Added Mac OS Troubleshooting

Just updated my Mac to latest OSX. Without `Command Line Tools` UPDATED it will just throw some crazy errors when compiling a demo. Trying to setup Android without `sudo` throws some crazy errors too.
Merge pull request #56 from AndreiRegiani/patch-5

Update 02-install-openfl.html.md