[README] Mujoco Requirement

[Carmezim]

Hi folks, first of all, thanks for the amazing material and work you open sourced, outstanding and generous gift to the community.

I wondered if would be interesting to add a side note regarding Mujoco 1.3 on recent Macs.
This version didn't support NVMe disks and still an open issue where there are more details regarding it.

[cbfinn]

Sure. Feel free to make a pull request for the README.

[Carmezim]

As soon as openai/gym#767 is merged we could remove the note for Mac users and update the dependencies to MuJoCo v1.5, if it's of your interest.

I tested those changes on an NVMe Mac and it works.
Until there if someone ends up here and wants to get it running:

https://github.com/openai/gym
cd gym
git pull origin pull/767/head
pip3 install -e .
pip3 install -U 'mujoco-py>=1.50.1'