alkimia: Add meta.longDescription

@7c6f434c wrote in [1]:

   Maybe copy-paste the upstream "Alkimia is the infrastructure for
   common storage and business logic that will be used by all financial
   applications in KDE. The target is to share financial related
   information over application bounderies." as longDescription for
   Alkimia? The current description makes me want to look up the
   homepage to find out what it actually is (then I do, and despair).

[1]: https://github.com/NixOS/nixpkgs/pull/39647#issuecomment-385169261



Signed-off-by: aszlig <aszlig@nix.build>