This is necessary for mounting SD cards from cameras, USB cameras, or USB phones to share their photos over a local WiFi network to my iPhone without a laptop or internet access.
See the following blog for links to Raspberry Pi Forum posts explaining the process of version 1. The first one was pickpocketed from me on a Hainan Airlines flight from Amsterdam to Beijing. While in China, I built version 2 with a built-in battery!
Up to Left Belt Clip