ZOOKEEPER-2364: "ant docs" fails on branch-3.5 due to missing releasenotes.xml.

[Chris Nauroth]

Could someone please help review ZOOKEEPER-2364?  I consider this a blocker for 
3.5.2-alpha, because it blocks our ability to update the docs, and we have 
several documentation patches to ship in 3.5.2-alpha.  Thank you.

--Chris Nauroth