added phone stand STL

Adds phone stand prototype for showing off phones at conferences, based off of pmOS logo, see #11 (closed).

Let me know if any changes need to be made with the design, I kinda just eyeballed everything (right now traveling, so don't have access to my caliper or anything). It's also likely you'll need to resize the model before printing, because STL files don't carry unit measurements and Blender tends to use meters as the unit.

Just out of curiosity, is it possible to email merge requests on this repository? This is only my second time making an MR on gitlab, and I tried doing it via email and a patch file for the first time, but gitlab rejected it.