Make a note in README that the Python3 port is ongoing.

README.md

@@ -25,3 +25,10 @@ Help
 
 There is a #dulwich IRC channel on Freenode, and a dulwich mailing list at
 https://launchpad.net/~dulwich-users.
+
+Python3
+-------
+
+The process of porting to Python3 is ongoing. Please not that although the
+test suite pass in python3, this is due to the tests of features that are not
+yet ported being skipped, and *not* an indication that the port is complete.