So because Android can be kind of annoying sometimes for dev work, especially if you're just interested in working on the aarch64 JIT or GLES code, booting a Linux distro can be pretty useful. This guide can also be extended to outright installing Ubuntu on the Shield TV by flashing the boot partition but we won't cover that. Also note, everything about this guide is non-destructive, so unless you decide to flash anything, you can't break your Shield TV.
I'm writing this guide assuming you have a functional UNIX environment of some sort and a non-pro Nvidia Shield TV 2015. The pro variant WILL NOT WORK with this specific guide, but the boot files are given for them as well in the XDA links below.
You'll need a USB device or MicroSD at least 4 GB large. 8 GB or higher is highly recommended.
Go download all the files here: https://drive.google.com/drive/folders/0B39Ag2JMI49DcDJUTnBlRUJPRzA