New 'Writing a Compiler Backend' document

Hi All,

I'm happy to say that Mason Woo took the existing "how to write a compiler backend" and completed it. It is now an extremely useful reference for people starting a new backend and/or hacking on existing ones. I really want to thank Mason for doing this, and if you have questions or comments, please let him know!

Here's a link:
http://www.llvm.org/docs/WritingAnLLVMBackend.html

-Chris