Adds a screenshot of rotating ‹three-planet› element to main README (#1)

README.md

@@ -25,11 +25,15 @@ The `<three-*>` web components can be used as follow (see also [demos/three-plan
 <three-app fps="24" camera="c01" scene="s01">
   <three-camera id="c01" type="perspective"></three-camera>
   <three-scene id="s01">
-    <three-planet id="earth"></three-planet>
+    <three-planet id="earth" animate></three-planet>
   </three-scene>
 </three-app>
 ```
 
+<a href="demos/assets/screenshots/20190222-three-planet-animated.png">
+  <img height="350" title="Screencopy of animated ‹three-planet› element"
+  src="demos/assets/screenshots/20190222-three-planet-animated.png"></a>
+
 ## Contributing
 
 This project uses [Lerna](https://lernajs.io) to manage the packages of this mono-repository. You can execute `npx lerna …` to run any Lerna command on the packages.
@@ -61,4 +65,4 @@ Start the local HTTP dev server and visit http://localhost:8081/demos/:
 
 See model in [design intentions](DESIGN.md), it shows some of the initial ideas.
 
-When creating an app, you would generally create one of more new Web Components, based on the [`ThreeObject`](packages/three-object/three-object.js) class — which itself inherits from `LitElement` — and add it to a scene. See the [packages/three-planet](packages/three-planet/three-planet.js) package for an example.
+When creating an app, you would generally create one of more new Web Components, based on the [`ThreeObject`](packages/three-object/three-object.js) class — which itself inherits from `LitElement` — and add it to a scene. See the [packages/three-planet](packages/three-planet/three-planet.js) package for an example.