Add documentaton for dataset README.md files
@lhoestq hmm - ok thanks for the answer.
To be honest I am not sure if this issue can be closed now.
I just wanted to point out that this should either be documented or linked in the documentation.
If you feel like it is (will be) please just close this.
Hi,
the dataset README files have special headers.
Somehow a documenation of the allowed values and tags is missing.
Could you add that?

Just to give some concrete questions that should be answered imo:
- which values can be passted to multilinguality?
- what should be passed to language_creators?
- which values should licenses have? What do I say when it is a custom license? Should I add a link?
- how should I choose size_categories ? What are valid ranges?
- what are valid task_categories?

Thanks
Philip
Add documentaton for dataset README.md files 
 Hi,
the dataset README files have special headers.
Somehow a documenation of the allowed values and tags is missing.
Could you add that?

Just to give some concrete questions that should be answered imo:
- which values can be passted to multilinguality?
- what should be passed to language_creators?
- which values should licenses have? What do I say when it is a custom license? Should I add a link?
- how should I choose size_categories ? What are valid ranges?
- what are valid task_categories?

Thanks
Philip 
 @lhoestq hmm - ok thanks for the answer.
To be honest I am not sure if this issue can be closed now.
I just wanted to point out that this should either be documented or linked in the documentation.
If you feel like it is (will be) please just close this.