Update documentation

Update both README and man page to mention the new -i, --interactive
option of gfsec-split.

README.md

@@ -82,7 +82,8 @@ will be unable to fetch the shares.
 
 As a convenience, before splitting you may call _gfsec-split_ with the
 single `-l` option to list the available external volumes and their
-identifiers.
+identifiers. Alternatively, the `-i` option may be used to both list
+the available volumes and select those to use in an interactive menu.
 
 
 gfsec-use

man/gfsec-split.1.in

@@ -11,6 +11,7 @@ gfsec-split \- Split a file into shares and dispatch them
 .IR N ]
 .RB [ \-c | --config
 .IR file ]
+.RB [ \-i | --interactive ]
 .RB [ \-k | --keep ]
 .RB [ \-l | --list-supports ]
 .B file
@@ -46,6 +47,10 @@ the secret with
 ) to the specified file. Default is the basename of
 the splitted file in $XDG_CONFIG_HOME/gfsecret.
 .TP
+.BR -i ", " --interactive
+Present the user with an interactive menu to specify the
+shares to create.
+.TP
 .BR -k ", " --keep
 By default,
 .B gfsec-split