Add description and document -

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>

doc/man/_description.txt

+**zathura** displays the given files. If a single hyphen-minus (-) is given as
+file name, the content will be read from the standard input. If no files are
+given, an empty **zathura** instance launches.

doc/man/zathura.1.rst

@@ -6,6 +6,11 @@ Synopsis
 
 .. include:: _synopsis.txt
 
+Description
+-----------
+
+.. include:: _description.txt
+
 Options
 -------