Add AppStream metadata (resolves #99).
Update changelog.
Add notice about AppStream data into documentation

Originally submitted at:

https://github.com/wincent/Command-T/pull/100

Includes a tiny typo fix.

Signed-off-by: Greg Hurrell <greg@hurrell.net>
Merge branch 'voxik/appstream'

Originally submitted at:

  https://github.com/wincent/Command-T/pull/100

Includes:

* voxik/appstream:
  Add notice about AppStream data into documentation
  Update changelog.
  Add AppStream metadata (resolves #99).

Signed-off-by: Greg Hurrell <greg@hurrell.net>