Feature request: Document supported GPUs for Docker.train

[JRMeyer]

We can point to the requirements for the base NVIDIA docker image, and give people an idea of what should work and what definintely won't work:

From NVIDIA's TF container release notes (aka, our base image for training):

GPU Requirements Release 20.06 supports CUDA compute capability 6.0 and higher. This corresponds to GPUs in the NVIDIA Pascal, Volta, Turing, and Ampere Architecture GPU families. Specifically, for a list of GPUs that this compute capability corresponds to, see CUDA GPUs. For additional support details, see Deep Learning Frameworks Support Matrix.