Add link to 'get tree recursively'

Add a note to the 'Get repo contents' section mentioning that to get a content listing recursively (not possible with 'contents'), one can use the repo's tree instead.
This is a bit difficult to find if one does not know the API, so should be worth noting here.
Merge pull request #213 from n3v1k/patch-1

Add link to 'get tree recursively'