http://oss-docs.addthiscode.net/hydra/latest/user-guide/
They took a bit longer get ready than the code itself, but the pretty 
hydra documentation is now part of the open source repo and being 
published.  Docs are if anything even harder than code, so feedback or 
suggestions for them is particularly welcome.
Also if you were wondering what all of those special javadoc tags are 
for, your answer has arrived: 
http://oss-docs.addthiscode.net/hydra/latest/user-reference/  Some very 
cool custom work builds reference docs specifically for each pluggable 
type (filters, query ops, etc).