add TROUBLESHOOTING.md

[ianfixes]

@hlovdal and @scls19fr in particular

I'm going through the list of issues / feature requests, and trying to compile a list of all the things that aren't very well documented and/or make for confusing error messages.

Right now, my list contains things like:

• examples compiling on platforms that you don't want them to
• compilation errors related to having the arduino_ci directory inside your project directory
• Symlinks or other problems in the Arduino library directory
• bundle install / execution problems, etc

if it's still fresh in your mind, can you list off any other strange errors that you ran into? I will include all those in my writeup.