appdata handling

[Markus Mayer]

A new version of a package I maintain added an appdata.xml. As I haven't 
handled such files before, I looked up the internet for some 
information. The only helpful hint I found, was a commit adding appdata 
support to the qt-creator package in fedora git. [1]

As more packages will add appdata file upstream, packagers will need 
information how to handle this within fedora. I suggest to these 
information to the Packaging Guidelines.

What I'm interested in is:
- Directory, Name, ownership and permissions for appdata.xml files
- %post/%postun scriptlets (if needed)
- If appdata-validate must be run during package build
- How long does it take that the new appdata is propagated to gnome-software

As a side note: build.log contains the following error:
error: Couldn't exec /usr/lib/rpm/appdata.prov: No such file or directory

[1] 
http://pkgs.fedoraproject.org/cgit/qt-creator.git/commit/?h=epel7&id=812bac7be961483c992a3337fc8a35c524c73079