added dutch_social
The Dutch social media tweets dataset. Which has a total of more than 210k tweets in dutch language. These tweets have been machine annotated with sentiment scores (`label` feature) and `industry` and `hisco_codes`

It can be used for sentiment analysis, multi-label classification and entity tagging
@lhoestq 
Updated the `dummy_data.zip `(<10kb)I had to reduce it to just a few samples. 
Train-Test-Dev  (20-5-5 samples) 

But the push also added changes from other PRs (probably because of a rebase!) So the files changed tab shows 466 files were changed!  
 Thanks ! The dummy data are all good now :) 

Looks like this PR includes changes to many other files than the ones for dutch_social now.

Can you create another branch and another PR please ? > 
> Can you create another branch and another PR please ?
@lhoestq 

I did a rebase. Now it doesn't include the other files. Does that help? 

 Yes thanks !