[seahorse-nautilus] README: update (and use Markdown)

[Niels De Graef <nielsdg src gnome org>]

commit 818afcb18942a049a819ce1bac4eef7348f566fa
Author: Niels De Graef <nielsdegraef gmail com>
Date:   Mon Dec 4 15:08:46 2017 +0100

    README: update (and use Markdown)

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+# Seahorse Nautilus extension
+The Seahorse Nautilus extension is an extension for
+[Nautilus](https://wiki.gnome.org/Apps/Nautilus) (also known as Files) which
+allows encryption and decryption of OpenPGP files using GnuPG.
+
+## Building
+You can build and install the Seahorse Nautilus extension using
+[Meson](http://mesonbuild.com/):
+```sh
+meson build
+ninja -C build
+ninja -C build install
+```
+
+## Issue tracker
+Seahorse-Nautilus uses the GNOME Bugzilla, where you can check the
+[list of open bugs](https://bugzilla.gnome.org/browse.cgi?product=seahorse) of
+Seahorse with the "Nautilus" component.
+
+If you'd like to report a bug in Seahorse-Nautilus or request an enhancement,
+please file an issue using the
+[appropriate form](https://bugzilla.gnome.org/enter_bug.cgi?product=seahorse&component=Nautilus).
+
+In case of a bug, please also add reproducible steps and your version of the
+Seahorse Nautilus extension.
+
+## Contributing
+If you would like to contribute a patch, you should send it in to the GNOME
+Bugzilla as well. If the patch fixes an existing bug, add the patch as an
+attachment to that bug report; otherwise, enter a new bug report that describes
+the patch, and attach the patch to that bug report.
+
+For more information on the recommended workflow, please read
+[this wiki page](https://wiki.gnome.org/Git/WorkingWithPatches).
+
+## More information
+Seahorse and its derivatives (such as the Seahorse Nautilus extension) has its
+own web page on https://wiki.gnome.org/Apps/Seahorse.
+
+To discuss issues with developers and other users, you can subscribe to the
+[Seahorse mailing list](https://mail.gnome.org/mailman/listinfo/seahorse-list)
+or join [#seahorse](irc://irc.gnome.org/seahorse) on irc.gnome.org.
+
+## License
+The Seahorse Nautilus extension is released under the GPL. See COPYING for more
+info.