Add stackoverflow tag to README.md and CONTRIBUTING.md

[dmigo]

As discussed on #3762

https://stackoverflow.com/questions/tagged/apache-superset - superset tag on stackoverflow

[coveralls]

Coverage remained the same at 71.271% when pulling cac0a5c on dmigo:stackoverflow into 5c03167 on apache:master.

[xrmx]

Given that superset is already in use i'd stick to that instead of apache-superset
https://stackoverflow.com/tags/superset/info

[dmigo]

@xrmx in general, I agree.
The only problem is that this tag is quite often misused, due to the ambiguity of the name.

[xrmx]

Better a confusing community that an empty one, no? :)

[dmigo]

Haha =) True.
But as far as I know it should be possible to add the tag as an alias.