Yes, it is this file! We can even host it on our b2g-installer repo if you give it to me
So this field in the JSON file lists all the upstream Android releases we can get blobs from. So you are right, you should list ALL the Android releases from Fairphone that works.
It means that you need to ensure all the builds listed here works. We are reusing the blobs (non free/non redistribuable components), and it might be possible there are incompatibilities. I would recommend that you test only the latest update available, and we can always point people to upgrae their phone before flashing.
If you have a device with a build that is not listed, and you plug it in the addon, it will not be listed/detected.
PR means you do the needed changes to builds.json, and push it to a branch, then open a pull request on Github. Please refer to Github's doc for this.
Also, please test your PR by changing the builds.json URL pref: this is handled at https://github.com/mozilla-b2g/b2g-installer/blob/master/about.js#L41 so you just have to open |about:config| in your firefox , create a new string pref named email@example.com and set the value to your PR's json file (the file, and not a link to the github page, otherwise it will fail).
Once your have changed that pref and restarted your firefox, plugging your Fairphone2 with a supported Android build you should be offered to flash B2G with your ROM