updates for 3.6 March 2020

doc/Guidebook.mn

@@ -24,7 +24,7 @@
 .ds vr "NetHack 3.6
 .ds f0 "\*(vr
 .ds f1
-.ds f2 "January 27, 2020
+.ds f2 "March 4, 2020
 .
 .\" A note on some special characters:
 .\" \(lq = left double quote
@@ -5164,6 +5164,8 @@ and a few bug fixes.
 NetHack 3.6.5 was released on January 27, 2020 containing some security fixes
 and a small number of bug fixes.
 .pg
+NetHack 3.6.6 was released in March 2020 containing a security fix.
+.pg
 The official NetHack web site is maintained by \fBKen Lorber\fP
 at
 .UR https://www.nethack.org/ .