README: Rewrite the Data Sources section, further minor tweaks
README: Rewrite the Data Sources section, further minor tweaks

The section is now more conceptual, to better reflect rising queries
about domain adaptation.

Martinfx/yodaqa,Martinfx/yodaqa,vineetk1/yodaqa,vineetk1/yodaqa,vineetk1/yodaqa,vineetk1/yodaqa,vineetk1/yodaqa,Martinfx/yodaqa,Martinfx/yodaqa,vineetk1/yodaqa,Martinfx/yodaqa,Martinfx/yodaqa