Add a usage section to readme

Author: marijnh

README.md

@@ -16,3 +16,20 @@ 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, keymap} from "@codemirror/view"
+import {standardKeymap, selectLine} from "@codemirror/commands"
+
+const view = new EditorView({
+  parent: document.body,
+  extensions: [
+    keymap.of([
+      ...standardKeymap,
+      {key: "Alt-l", mac: "Ctrl-l", run: selectLine}
+    ])
+  ]
+})
+```