Add docs on default values
Add docs on default values

Closes #29.
pakoito/dart,AbnerLv/dart,AbnerLv/dart,f2prateek/dart,bryant1410/dart,f2prateek/dart,MaTriXy/dart,MaTriXy/dart,pakoito/dart,brunodles/dart,brunodles/dart,bryant1410/dart