housekeeping updates for 3.6.4

doc/Guidebook.mn

@@ -24,7 +24,7 @@
 .ds vr "NetHack 3.6
 .ds f0 "\*(vr
 .ds f1
-.ds f2 "December 5, 2019
+.ds f2 "December 17, 2019
 .
 .\" A note on some special characters:
 .\" \(lq = left double quote
@@ -45,6 +45,8 @@ A Guide to the Mazes of Menace
 .au
 Original version - Eric S. Raymond
 (Edited and expanded for 3.6 by Mike Stephenson and others)
+.sp 1
+\*(f2
 .hn 1
 Introduction
 .pg
@@ -5149,6 +5151,9 @@ participant for decades, joined the NetHack Development Team in late May 2019.
 NetHack 3.6.3 was released on December 5, 2019 containing over 190 bug
 fixes to NetHack 3.6.2.
 .pg
+NetHack 3.6.4 was released on December 17, 2019 containing a security fix
+and a few bug fixes.
+.pg
 The official NetHack web site is maintained by \fBKen Lorber\fP
 at
 .UR https://www.nethack.org/ .

doc/Guidebook.tex

@@ -45,7 +45,7 @@
 %.au
 \author{Original version - Eric S. Raymond\\
 (Edited and expanded for 3.6 by Mike Stephenson and others)}
-\date{December 5, 2019}
+\date{December 17, 2019}
 
 \maketitle
 
@@ -5753,6 +5753,11 @@ for decades, joined the {\it NetHack Development Team} in late May 2019.
 NetHack 3.6.3 was released on December 5, 2019 containing over 190 bug
 fixes to NetHack 3.6.2.
 
+%.pg
+\medskip
+NetHack 3.6.4 was released on December 17, 2019 containing a security fix and
+a few bug fixes.
+
 %.pg
 \medskip
 \nd The official {\it NetHack\/} web site is maintained by {\it Ken Lorber} at