Document fix for issue #2013.

Renci.SshClient/Build/nuget/SSH.NET.nuspec

@@ -34,6 +34,7 @@
           * SftpClient constructor throws ArgumentException when host contains underscore (issue #1845)
           * Signing key is missing from source download (issue #2455)
           * Forwarded Port channels can get stuck waiting (issue #1558)
+          * NullReferenceException when SftpFileStream is finalized after dispose of session (issue #2013)
 
           2014.4.6-beta1
           ==============