Update AppStream file to meet FlatHub requirements

[finn-wa]

The latest FlatHub build failed due to some lint errors on the AppStream file: https://buildbot.flathub.org/#/builders/7/builds/15587/steps/11/logs/stdio

These were the required fixes:

• Add developer tag (https://docs.flathub.org/docs/for-app-authors/linter/#appstream-missing-developer-name)
• Shorten the summary to be less than 35 characters. Someone might want to suggest something more exciting than "Karaoke for your PC", but 35 characters isn't much to work with! (https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#summary)
• Add captions to screenshots (https://docs.flathub.org/docs/for-app-authors/linter/#appstream-screenshot-missing-caption)
• Rename recommended tag to recommends and remove "keyboard" from supports. Each value can only appear once across recommends, supports, and required. (https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-relations)

Also I noticed that there was an issue for documenting the FlatHub release process, so I added a little note in there to contribute to that. I don't know anything else about it though, as this was my first attempted FlatHub release.

[barbeque-squared]

I've linked #803 so that Github will actually auto-close it, because looking at the diff this should be enough to cover that issue. I guess we'll find out next month!