Pull Contributing Guidelines and Requirements into CONTRIBUTING.md
Add link to CONTRIBUTING.md in README
Remove point about squashing commits.
Validate Server method input. Addresses BUKKIT-3687

CraftServer methods that implement the Server interface will throw an
IllegalArgumentException if a method cannot operate on a null input
and given a null pointer.

This causes methods to fail early and identify that a plugin is
responsible for passing in an invalid argument. This will only
change the exception thrown, if there originally was a thrown
exception. This helps with hunting down legitimate problems
with CraftBukkit.
BlockState for Command Blocks.  Adds BUKKIT-3805.
Readd missed diff for minecart container size. Fixes BUKKIT-3826
Don't filter out -1 data in ItemStack. Fixes BUKKIT-3824