include docs directory in sdist

--HG--
extra : transplant_source : %08%E9%18%14%FC%D4vh%E5%83%B8fk%AE%C17%1D%9C%C8%5B

MANIFEST.in

@@ -21,6 +21,7 @@ recursive-include Cython/Debugger/Tests *
 recursive-include Tools *
 recursive-include tests *.pyx *.pxd *.pxi *.py *.h *.BROKEN bugs.txt
 recursive-include tests *_lib.cpp *.srctree
+recursive-include docs *
 include runtests.py
 
 include Cython/Debugger/Tests/cfuncs.c