Add a "resources" section to the front page (#215)

With links to
  - MaterialX web viewer running OpenPBR default material
  - OpenPBR-viewer project and web app

Author: portsmouth

README.md

@@ -18,6 +18,12 @@ OpenPBR Surface is an open standard hosted by the [Academy Software Foundation](
 * **[Reference implementation](reference/open_pbr_surface.mtlx)** – written in [MaterialX](https://materialx.org/)
 * **[BibTeX citation](openpbr.bib)**
 
+### Resources
+
+* [MaterialX Web Viewer](https://academysoftwarefoundation.github.io/MaterialX/?file=Materials/Examples/OpenPbr/open_pbr_default.mtlx) - WebGL rasterization renderer using MaterialX [implementation](reference/open_pbr_surface.mtlx) of OpenPBR
+* [OpenPBR-viewer](https://github.com/portsmouth/OpenPBR-viewer) - self-contained example implementation in a WebGL pathtracer (run [here](https://portsmouth.github.io/OpenPBR-viewer))
+* [#openpbr](https://academysoftwarefdn.slack.com/channels/openpbr) - public Slack channel for discussions, hosted by ASWF
+
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 [![License: CC BY-SA 4.0](https://img.shields.io/badge/License-Apache%202.0-informational.svg)](LICENSE)