Complementary informations for GPU users

Great job with the otftf docker suite, I grabbed the otbtf2.0:gpu docker and it really eases things up! By the way, some additional information for gpu users might be included in the documentation, basically:

• the need to install the nvidia-container-runtime package at the host level
• the need to specify the '--gpus' option when calling the container to make system gpus visible in the docker

Cheers! Raf