updates for 0.12.1 release

- updated readme
- updated credits
- updated changelog

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
changed sparkel appcast feed to own domain

avoid using github for appcast storage.

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
update screenshot, changelog

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>