feature request from issue #241: AppData descriptor

Created this AppData descriptor as per the instructions on http://people.freedesktop.org/~hughsient/appdata/. I put it in ./share/freedesktop.org under the project root; I wasn't sure where else to put it. I added a README file to explain.

I also modified CMakeLists.txt so that it is installed at /usr/share/appdata during "make install". I tested the install and it works, but my desktop is XFCE and so doesn't use these files. I tested the XML and it was valid. Feel free to modify the file, I just wanted to get it started.

I also added our nicknames to the AUTHORS file.