Starts on a list of external deps required for dev
I've found that I need to install MeCab manually on my AS Mac while working on #18702.

# What does this PR do?

Adds nudge to install MeCab from Homebrew to dev contributing instructions.

## Before submitting
- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
-  [X] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [X] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Documentation: @sgugger


_The documentation is not available anymore as the PR was closed or merged._