Add a usage section to readme

Author: marijnh

README.md

@@ -16,3 +16,28 @@ We aim to be an inclusive, welcoming community. To make that explicit,
 we have a [code of
 conduct](http://contributor-covenant.org/version/1/1/0/) that applies
 to communication around the project.
+
+## Usage
+
+```javascript
+import {EditorView} from "@codemirror/view"
+import {autocompletion} from "@codemirror/autocomplete"
+import {jsonLanguage} from "@codemirror/lang-json"
+
+const view = new EditorView({
+  parent: document.body,
+  extensions: [
+    jsonLanguage,
+    autocompletion(),
+    jsonLanguage.data.of({
+      autocomplete: ["id", "name", "address"]
+    })
+  ]
+})
+```
+
+This configuration will just complete the given words anywhere in JSON
+context. Most language modules come with more refined autocompletion
+built-in, but you can also write your own custom autocompletion
+[sources](https://codemirror.net/docs/ref/#autocomplete.CompletionSource)
+and associate them with your language this way.