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

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave approved this pull request.

This is pretty neat! LGTM w/ a minor comment.

> 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.

Please do! CE is easily approachable for most people.

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