Allow collaborators to self-assign issues
What do you think, @lhoestq? 😉 

I think this could be another step to facilitate community contributions. @lhoestq, it doesn't exist in `transformers`... I picked the idea from `scikit-learn`, where I have previously collaborated...

And sure, this must be documented! I just wanted first to know your opinion...
Allow collaborators (without write access to the repository) to self-assign issues.

In order to self-assign an issue, they have to comment it with the word: `#take` or `#self-assign`.