Let's try to answer Frequently Asked Questions...

Raspberry Pi 1 B or B+, Raspberry Pi Zero, Raspberry Pi 2 and Raspberry Pi 3
You can make a donation :

You can create issues on the recalboxOS GitHub page, if you encounter any https://github.com/recalbox/recalbox-os/issues.
You can participate on the recalbox.com forums
You can participate on the wiki https://github.com/recalbox/recalbox-os/wiki
We try to add a maximum of free games for you to have a full recalbox experience from the first boot!
Rendez-vous on https://forum.recalbox.com/topic/52/homebrews-sur-la-recalbox to share your homebrews!
RecalboxOS has been designed to ease the integration of the system into Bartops and Arcade Cabinets.
You can directly connect you controls on the RPi GPIOs, avoiding the addition of an USB controller.
You can deactivate several menu options to avoid unwanted settings. You can also use different video outputs like HDMI/DVI/VGA (with adapters) or RCA.Get more information on the bartop page of the wiki.
Only systems with avaible games are displayed in EmulationStation. Try to add your roms (see the Manual) and reboot your recalbox to see the new systems.
The option is available in the Sound Settings -> FRONTEND MUSIC
It's a reported issue. For now, the PS3 controllers are only supported via bluetooth.
On the recalbox-os github, you can refer to "issues" wich are the current development tasks.If you refer to recalbox "milestones" you will see closed and currently developed tasks for each recalbox version.
On Raspberry Pi 1 and Zero, we highly recommend to overclock to EXTREME mode. You will void the warranty of the RPi, but the performance upgrade is really impressive!
We are working on new systems integration, but you will have to wait for the 4.1 release.
We are working on it and we hope we can add the embedded bluetooth support for RPi3 soon.