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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Added the MLIR links and split language and backend resources.

TableGen is live in Compiler Explorer now, but I haven't added a "trunk" variant yet. So I'll add links for that in a follow up when that's there.

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