Add manpage

Closes #259

youtube-dl-gui.1

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+.\" [program name] [section] [date YYYY-MM-DD] [version] [empty]
+.TH YOUTUBE\-DL\-GUI 1 "2018-01-13" "Version 0.4" ""
+
+.SH NAME
+youtube\-dl\-gui \- cross platform graphical user interface for youtube\-dl.
+
+.SH SYNOPSIS
+.B youtube\-dl\-gui
+
+.SH DESCRIPTION
+Youtube\-dl\-gui is a graphical frontend of the popular youtube\-dl
+command\-line program. As such, it does not take any command\-line parameters.
+You may configure the program through the graphical settings dialog, or the
+configuration file.
+
+.SH FILES
+.\" .IP text indent_size (.IP = Indented Paragraph)
+.IP "\fB$HOME/.config/youtube\-dlg/settings.json\fR" 4
+Configuration file.
+.\" .IP text indent_size (.IP = Indented Paragraph)
+.IP "\fB$HOME/.config/youtube\-dlg/log\fR" 4
+Log file.
+
+.SH NOTES
+FAQS: https://github.com/MrS0m30n3/youtube-dl-gui/blob/master/docs/faqs.md
+
+.SH SEE ALSO
+.BR youtube-dl (1)