@kku of the Isotammi project has created a 5.2 addons-en.json listing and the companion 5.2 addon repository for testing their add-ons and the new Addon Manager. (Another big ‘thank you’ to Kari!)
https://raw.githubusercontent.com/Taapeli/isotammi-addons/master/addons/gramps52
In the following dialog, I have added a listing (See the + - buttons in the lower left of the dialog) for the Isotammi collection and a test list on my local drive. (Note the triple slash for the local file:/// path. There is no browsing option to the location for a hosted or local path. And it fails silently. You have to get it exactly right or be baffled.)
During the 5.2 beta cycle, the format of the addon list could evolve quickly. So until the public release marks the format has stopped changing, Paul Culley is maintaining a mirror compatible with the evolution.