Adding README.md for Kinemage extension

src/extensions/kinemage/README.md

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+# Kinemage extension
+
+This extension adds support for the Kinemage molecular graphics format based on the
+[kinemage](http://kinemage.biochem.duke.edu/static/files/PDFs/format-kinemage.pdf) format specification.
+
+It currently supports the following features:
+- Drag-and-drop of Kinemage files into the display area
+- Display of @ball, @sphere, @vector, @dot, @ribbon, and @triangle objects
+- Hovering over objects to see their labels (if present)
+
+Currently unsupported features include:
+- master and submaster selections of visible objects
+- Setting one or more viewpoints
+- animations
+- @label and @ring objects
+- @hsvcolor keyword for coloring by hue, saturation, and value
+- 'fore' and 'rear' keywords for different front and back colors
+
+Current limitations include:
+- Lines and triangles are a single color, not colored by vertex
+- Line segments in Mol* do not support end-caps for wide lines, so there are artifacts in highly-curved lines
+- The default perspective view and white background for Mol* differs from that of Kinemage