Add space under front-page caption (#131)

Adds a space under the image/caption, which helps to visually separate the caption from the paragraph.

README.md

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 <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>
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 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.