Hello world in gtkmm, Chapter 3. Basics

[Kyle Flanagan <kyleflanagan gmail com>]

Perhaps it was just me being a novice programmer, but the fact that you 
need to compile main.cc and helloworld.cc together wasn't clear. Perhaps 
a note could be made about this, or main.cc could include helloworld.cc. 
Just my two cents on hopefully making it easier for novice programmers 
to begin using gtkmm.