Merge part of rev 29784 from 3.3 branch.

This is news about ZEO protocol changes.  The promised
update of README.txt will have to be rewritten from
scratch for 3.4 (the new text in 3.3's README.txt isn't
quite right for the 3.4 release).

NEWS.txt

@@ -151,6 +151,17 @@ make a connection.  The ZEO reconnection tests may run much faster now,
 depending on platform, and should suffer far fewer (if any) intermittent
 "timed out waiting for storage to connect" failures.
 
+ZEO protocol and compatibility
+------------------------------
+
+ZODB 3.3 introduced multiversion concurrency control (MVCC), which required
+changes to the ZEO protocol.  The first 3.3 release should have increased
+the internal ZEO protocol version number (used by ZEO protocol negotiation
+when a client connects), but neglected to.  This has been repaired.
+
+Compatibility between pre-3.3 and post-3.3 ZEO clients and servers remains
+very limited.  See the newly updated ``Compatibility`` section in
+``README.txt`` for details.
 
 FileStorage
 -----------