Adding Core Maintainer List to CONTRIBUTING.md
@lhoestq there is no per-module maintainer and the list is me alone nowadays ^^' @lhoestq 
Oh... I feel for you. 
What are your criteria for choosing a core maintainer? 
It seems like it's too much work for you to manage all this code by yourself.

Also, if you don't mind, can you check this PR for me?
#7368 I'd like this to be added as soon as possible because I need it.
### Feature request

I propose adding a core maintainer list to the `CONTRIBUTING.md` file.

### Motivation

The Transformers and Liger-Kernel projects maintain lists of core maintainers for each module. 
However, the Datasets project doesn't have such a list. 

### Your contribution

I have nothing to add here.