add README.md

packages/editor/README.md

@@ -0,0 +1,14 @@
+# `@growi/editor`
+
+## Development with vite
+
+```bash
+cd packages/editor
+yarn serve
+```
+
+## Playground
+
+If you need to simulate something `@growi/app` feature, you can implement it into the playground.  
+See the `components/playground` directory.
+