added Revit three.js JSON exporter RvtVa3c

utils/exporters/revit/README.md

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+RvtVa3c - Revit Three.js Exporter
+=================================
+
+RvtVa3c is a Revit add-in that exports the Revit model to a three.js version 4 JSON format.
+
+RvtVa3c is part of the [vA3C project](https://va3c.github.io), "an open source, browser-based 3D model viewer for AEC models that uses [three.js](https://github.com/mrdoob/three.js) to render 3D geometry in the browser, enabling authors in the AEC industry to easily publish their 3D design work on the web for free.
+
+RvtVa3c and its documentation live in their own [RvtVa3c GitHub repository](https://github.com/va3c/RvtVa3c).