Add note about pathlib.Path.

CONTRIBUTING.rst

@@ -39,7 +39,7 @@ bytestrings. It is up to the Dulwich API user to encode and decode them if
 necessary. The porcelain may accept unicode strings and convert them to
 bytestrings as necessary on the fly (using 'utf-8').
 
-* on-disk filenames: regular strings
+* on-disk filenames: regular strings, or ideally, pathlib.Path instances
 * git-repository related filenames: bytes
 * object sha1 digests (20 bytes long): bytes
 * object sha1 hexdigests (40 bytes long): str (bytestrings on python2, strings