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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> (by the way should this be curated and split into "TableGen" vs "some TableGen backends"?)

Yes, I see the two conflated all the time and fell into the trap myself.

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