Add title image credits

[virtualzavie]

Text used for caption:

Shader Playground, rendered in Arnold for Maya, using OpenPBR Surface. Artwork by Nikie Monteleone.

[portsmouth]

Suggest to put the image first, with a small centered caption below. This works:

# OpenPBR Surface

<p align="center">
<img src="images/OpenPBR title.jpg" title="OpenPBR demonstration scene" />
<sub><i>Shader Playground, rendered in Arnold for Maya, using OpenPBR Surface. Artwork by Nikie Monteleone.</i></sub>
</p>
<br>

A white paper specifying an über-shader that aims to provide a material representation capable of accurately modeling the vast majority of materials used in practical visual effects and feature animation productions.

[portsmouth]

While you're at it:

• change to über-shader instead of über-surface shader
• get rid of "typeset in Markdeep".