Hi, and welcome to the forum.
The currently released image has a relatively limited support for dongles, but the upcoming update will extend the support to more commonly found Realtek devices in addition to Atheros and Ralink dongles. Devices based on rtl8192cu driver should have a reasonably high chance of working once the update is released.
My personal favorite is the TP-Link TL-WN722N (Atheros-based). It has a decent signal with the big antenna, and the antenna can be replaced with an after-market one for even more range. It doesn't support that many clients, though. If you go for this one, be careful: (1) it is big, so it will make at least 1 USB port unusable (will cover it), and (2) there are similar models from TP-Link that look identical but are not based on Atheros (they do have different model number). Not sure what strength external antennas it can support nor its range rating, but this is probably the most flexible option. (In addition, most Atheros-based devices should work fine).
The one that was in the kit is a Ralink-based Tenda W311MI dongle (RT5370). It's mid-range in terms of price, with decent support on Linux, and that's why it was chosen. Another reason is that it can support more clients than the other dongles. I forgot where I got the numbers, but I can look it up if you're interested. Someone did a test with a few of the commonly available dongles. You may see dongles with a similar form-factor (body about the same size as USB plug) with red-grey "WiFi n" marking. They are usually based on the same chip. I've seen some that have a detachable antenna. For example this one.
Other common dongles are Netis WF2120 and Edimax EW-7811Un nano dongles, both based on Realtek radio, and supported in the (not-yet-released) update. These weren't supported till recently, so I had no reason to look too deeply into their characteristcs, and I'm not sure how good they are. You will also run into many nano dongles which have a similar form factor as Edimax and Netis nano dongles (body about half the size of the USB plug). 9 times out of 10, they are based on the same or similar Realtek radio as those two.
Note that power may become unstable if you add a dongle that is too power-hungry. I haven't had too many issues with the mentioned dongles using a 1.5A phone charger, but YMMV. I think @pradeeka here mentioned some issues using a 2A charger.