Document social media arguments in the man page

fgallery.1

@@ -137,13 +137,28 @@ Set the JPEG quality of the preview images to
 The default is 90.
 .It Fl -index Ar URL
 Specify the URL location for the index and back buttons.
+.It Fl -title Ar TITLE
+The title for your gallery, for social media.
+.It Fl -description Ar DESCRIPTION
+A description of your gallery, for social media.
+.It Fl -url Ar URL
+The URL where you will install the gallery, for social media.
 .It Fl -link-orig[=TYPE]
 Change the method used to copy the original images.  If this option is
 not given, the method defaults to "copy".  If the type is not specified
 it sets the method to "hard".  Otherwise the type must be one of "copy",
 "hard", "soft" or "ref" - for normal copy, hard-linking, soft-linking or
 ref-linking respectively.
 .El
+.Ss Social Media
+Linking to your gallery on social media will not produce a nice
+preview unless you provide these three arguments: the title to use
+.Pq Fl -title Ns ,
+a short description
+.Pq Fl -description Ns ,
+and the entry URL
+.Pq Fl -url Ns .
+The first image in the gallery will be used as the preview image.
 .Sh SEE ALSO
 .Xr ImageMagick 1 ,
 .Xr exiftran 1 ,