[llvm] [llvm][TableGen] Add a README to the main TableGen folder (PR #69943)

[David Spickett via llvm-commits]

DavidSpickett wrote:

@joker-eph do you know if there's any other MLIR tablegen things worth mentioning? I've just linked to the LSP server for now. Maybe MLIR has a better page than that I should link instead.

https://github.com/llvm/llvm-project/pull/69943