Documentation: move RelNotes into a directory of their own

There are 108 of them already.  That's a bit more than one third of
all the files in the Documentation directory already, and still growing.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/install-webdoc.sh

@@ -6,7 +6,7 @@ for h in \
 	*.txt *.html \
 	howto/*.txt howto/*.html \
 	technical/*.txt technical/*.html \
-	RelNotes-*.txt *.css
+	RelNotes/*.txt *.css
 do
 	if test ! -f "$h"
 	then