adding moz-context-properties link to README

Author: chrisdavidmills

README.md

@@ -6,6 +6,8 @@ The "animation-frames-timing-function" directory contains a simple example that
 
 The "counter-style-demo" directory contains a demo for the [@counter-style documentation](https://developer.mozilla.org/en-US/docs/Web/CSS/@counter-style). See the live demo [here](https://mdn.github.io/css-examples/counter-style-demo/).
 
+The "moz-context-properties" directory contains a demo for the [-moz-context-properties documentation](https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-context-properties). See the live demo [here](https://mdn.github.io/css-examples/moz-context-properties/).
+
 The "editable-examples" directory contains CSS examples that are intended to be embedded in MDN pages as live editable samples.
 
 The "object-fit-basics" directory contains a simple page demonstrating typical usage of different <code>object-fit</code> and <code>object-position</code> values. [Run example live](http://mdn.github.io/css-examples/object-fit-basics/).