Update README.md

[vikashplus]

@Vittorio-Caggiano A few thoughts

• pictures are a bit uninviting at the moment. Can we find better pictures?
• lets arrange them in a table
• In model description provide few little details such as #dof and #muscles in the models
• Lets make sure the all the pics of the models are of same size.
• the background is not black. I use the github dark mode. The page looks weird with the pictures.

[Vittorio-Caggiano]

not sure about the background ... If I render in black it is still different

[vikashplus]

Lets go with black. Looks a lot better than the previous color.