Does the 2015 Retina MacBook have firmware target disk mode? I've read the user manual and FAQ on ports for the new 2015 Retina MacBook and can't tell if it supports target disk mode. 

Apple's article on target disk explicitly says Firewire and Thunderbolt are required for target disk mode but the excellent iMore review article says:


  You can transfer data at up to 5 Gpbs over USB 3.1, USB 3, or USB 2, or over ethernet with a USB adapter. (Including target disk mode.)


For the purposes of this question - target disk mode is exposing the internal MacBook storage using firmware and not the OS X installed on the computer itself.

What is needed to place this Mac in target disk mode and connect it to another computer that would presumably see it as a USB 3.1 Gen 1 hard drive?
Yes - you need an USB C data cable to use this and the power cable Apple ships isn't a data cable for these needs, but the firmware and hardware support target move over USB.

Apple's MacBook ports article now has a section titled "USB Target Disk Mode":


  USB Target Disk Mode
  Your MacBook with USB-C port supports USB Target Disk Mode. Enable Target Disk Mode for use with the following USB-C cables:
  1) USB 3.0 or USB 3.1 USB-A to USB-C Cable 
  Target Disk Mode can be used with this cable to manually transfer data from any Mac notebook with USB-A port(s) to your MacBook with USB-C port. This cable cannot be used to migrate data using the Setup Assistant or Migration Assistant application.
  2) USB 3.0 or USB 3.1 USB-C Cable (USB-C to USB-C)
  Target Disk Mode can be used to migrate data from any MacBook with a USB-C port to another MacBook with USB-C port using the Setup Assistant or Migration Assistant application. 
  Note: These cables are not currently available from Apple.
  To enable Target Disk Mode, hold down the T button on your keyboard while starting your MacBook. Then connect either USB-C cable described above. Note that the USB-C Charge Cable (2m) that comes with MacBook does not support Target Disk Mode.Your MacBook supports USB Target Disk Mode when you use it with a compatible, full-featured (“super speed”) USB-C cable. Note that the USB-C Charge Cable (included with MacBook) does not support USB Target Disk Mode.
  
  To enable Target Disk Mode, hold down the T button on your keyboard while starting your MacBook. Then connect the USB-C cable. Use a full-featured USB-C to USB-C cable to connect to another MacBook, or a full-featured USB-A to USB-C cable to connect to a Mac with standard USB-A ports.


Since this section was still available two days ago, and the MacBook launched on April 10, 2015 — I would say the answer is almost certainly yes, but not with the cable it comes with.