Modify README.md to include instructions ensuring GPU compilation on linux

[avandekleut]

I realized mujoco_py wasn't using the GPU for rendering. I had to do the following to ensure that it was rendering:

First, ensure that a folder matching the glob '/usr/lib/nvidia-[0-9][0-9][0-9]' exists. If it doesn't, then just fake one:

sudo mkdir /usr/lib/nvidia-000

Make sure that this is part of your LD_LIBRARY_PATH by adding this to your .bashrc file:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib/nvidia-000

Next, make sure that libGLEW.so is visible by adding this to your .bashrc file:

export LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libGLEW.so

Then reload the .bashrc file:

source ~/.bashrc

Finally, uninstall and reinstall mujoco_py

pip uninstall mujoco_py
pip install --no-cache-dir mujoco_py

Check whether it was install with GPU support:

import mujoco_py
mujoco_py.cymj

<module 'cymj' from '<path_to_mujoco_py>/mujoco_py/generated/cymj_2.0.2.13_37_linuxgpuextensionbuilder_37.so'>

ensure that it says cymj_2.0.2.13_37_linuxgpuextensionbuilder_37.so and not cpu.

[buoyancy99]

A PR has been created here