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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

We have our ODS definition docs: https://mlir.llvm.org/docs/DefiningDialects/AttributesAndTypes/  and https://mlir.llvm.org/docs/DefiningDialects/Operations/

This isn't generic "TableGen" but your "Resources for learning the language" aren't all generic either I believe (by the way should this be curated and split into "TableGen" vs "some TableGen backends"?)


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