Add document to commuity app section for FlipClock

[JaeSeoKim]

I'm made simple flip clock for MiyooMini series.

So I added the documentation to the Community Apps section of Onion.

_{https://github.com/JaeSeoKim/sdl-flip-clock}

[XK9274]

What a great project & lovely looking clock app - happy to add this to the docs. Have cloned and tested your docs page and all looks good. (The app also looks great!)

Unrelated to the PR but I'd recommend when you release to include the files already inside a FlipClock directory, people don't always read the readme