Add `Get it on pi-apps` badge to `README.md`

[AM7999]

Add [![badge](https://github.com/Botspot/pi-apps/blob/master/icons/badge.png?raw=true)](https://github.com/Botspot/pi-apps) to readme.md

[AM7999]

Will be making a PR for this

[AM7999]

Cant make a PR

[htv04]

I would add this, but given the current state of FNFR on the Raspberry Pi, I wouldn't want to advertise it until all its issues have been fixed.

[AM7999]

Alright then

[htv04]

Reopening because I do plan to add this when FNFR works properly on the Raspberry Pi.