As i looked for such a post here and couldn’t find one, and as there is no sub-category for these strange and slightly obsolete devices, here we go.
The Ambernic RG350 (and RG280, RG350P, RG351, etc) are a series of cheap gaming devices, architectured around the antiquated processor jz4770 dual core. They are interesting because 1/ they are cheap 2/ they run OpenDingux (a version of linux frozen in time) and not Android, which matters for tinkerers and google-haters alike 
Recently RetroArch got ported and optimized for the series of devices (yay!), and this thread aims at listing resources to help fellow RG-ists to find information about it.
Here is what i have found so far:
- An “official” teaser and an “official” announcement:
https://www.libretro.com/index.php/coming-soon-retroarch-for-opendingux-release-gcw-zerorg-350etc/ and https://www.libretro.com/index.php/retroarch-1-9-1-shines-brightly-on-opendingux/
- A pretty exhaustive guide by RetroGameCorps:
- Two videos (by the same) about the topic:
and
- Another video by MashTec:
- A complicated “installer” that compiles a .opk that is supposed to do all automatically (i personally preferred the manual approach, it’s simple enough):
- a bunch of “unofficial” cores:
- a nice “boot script” to boot the OpenDingux directly into retroarch, with automated save/load states that almost feel like a “deep sleep” and enable quick resume of last played game! <3
What is missing?