add tip to datasources.rst

Ran into this problem when coding a datasource to talk to a JSON API.  Especially because the example is for an API, it could be helpful to mention that find('first') (which is what I used out of habit) won't work-- you must use find('all').
Update datasources.rst
Merge pull request #636 from mechcozmo/patch-1

add tip to datasources.rst