I’m sure there are several ways to do this and hundreds of Linux distributions to choose from, but this is how I’ve done it for eOS:
1. Download eOS ISO file from > https://elementary.io They’d like you to make a donation to ongoing development, which I don’t have a problem with as I plan on trying to live with it for a time. If you’re just wanting to test things out, you can enter a value of £0 and still download.
2. Burned the ISO file to a blank DVD-R in macOS (simply right-click the ISO file and select Burn to Disc from the menu)
3. Booted MBA to new rEFInd menu with connected Apple SuperDrive and ISO disk inserted