Prepare release 0.6.4

Change-Id: I5f72c2fbf2a2ec86c108a47b4c854b069267276b
diff --git a/docs/release-notes/release-notes-0.6.4.rst b/docs/release-notes/release-notes-0.6.4.rst
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+NFD version 0.6.4
+-----------------
+
+Release date: October 19, 2018
+
+This minor release fixes a regression introduced in version 0.6.3 that prevents NFD from
+starting when RIB security configuration is enabled (:issue:`4759`).
+
+For details of new and improved features since version 0.6.2, refer to the :ref:`release notes
+of version 0.6.3 <v0.6.3>`.