Document distinction of YAML multiple documents vs sequence/array of values

[cowtowncoder]

(see #132 for background)

So: handling of a single physical document containing multiple logical documents is quite specific in that one has to use mapper.readValues() to deal with it, and can not use regular readValue() to bind as collections (for example). There are technical reasons for this difference but it is not necessarily clear from user perspective this is a different use case.

So this should be somehow documented. An immediate place is probably on README for YAML module, but perhaps something could be added to readValues() method too, to mention it does not just related to root-level JSON values (like "linefeed-separate JSON") but also more generally to some other data formats.