New TableGen document

As the first tentative step towards documenting the LLVM code generator
(and generally closing one of the massive holes in the dox), I just wrote
up the "TableGen Fundamentals" document, and uploaded it here:

This describes the basic idea behind TableGen, the type system and
expression syntax it uses, why it's useful, etc. If you are interested in
TableGen or the LLVM code generator, I would recommend taking a look at
it, then sending me feedback. :slight_smile:

The current plan is to continue to flesh out and expand this document,
probably through the inclusion of some more examples. After that, the
"grand unified LLVM code generator" document should describe how the
TableGen interfaces that the code generator requires, as well as how the
whole thing fits together.