message: Add some links on the introduction page
diff: # Introduction
  -Welcome to the Pikelet book!
  +Pikelet is a small dependently typed language. It doesn't do many interesting
  +things yet, but hopefully that will change in the future!
  +
  +- [Source code](https://github.com/brendanzab/pikelet)
  +- [Issues](https://github.com/brendanzab/pikelet/issues)
  +- [Gitter Chat](https://gitter.im/pikelet-lang/Lobby)
  +
  +```
  +id : (a : Type) -> a -> a;
  +id a x = x;
  +
  +const : (a b : Type) -> a -> b -> a;
  +const a b x y = x;
  +```
  