#76 - added documentation for extends option
Expand on extends option behaviour.

After trolling the spec for the best part to link to, it hit me that linking to the spec, or saying that it's in the spec, implies the user should look there. Given the format of specs in general, this sucks for the user to have to do.
Merge pull request #77 from skatejs/issue/#76-create-documentation-for-extends-option

#76 - added documentation for extends option