Example of dependency definition for both default and dev

[vphilippon]

Adding the example given in #91 by @kennethreitz

I originally wanted to move "e1839a8" = {path = ".", editable = true} to the dev section, and adding the "real package dependency" in the default section, but I'm not sure how to make it clear those 2 are linked, so I skipped it for now. If someone has an idea for this one, go ahead.