Create a file to hold differences to other software

Also, include one known difference to bind9. This is both to store it
somewhere before we forget about it and to have an example to how the
file would look like.

doc/Makefile.am

 SUBDIRS = guide
 
-EXTRA_DIST = version.ent.in
+EXTRA_DIST = version.ent.in differences.txt
 
 devel:
 	mkdir -p html

doc/differences.txt

+Differences of Bind 10 to other software
+========================================
+
+Bind 9
+------
+
+TODO: There are definitely more differences than just this.
+
+* When an incoming zone transfer fails, for example because the
+  received zone doesn't contain a NS record, bind 9 stops serving the
+  zone and returns SERVFAIL to queries for that zone. Bind 10 still
+  uses the previous version of zone.