Fill in the readme.

Author: sjrd

README.md

@@ -2,4 +2,118 @@
 
 A [Vite](https://vitejs.dev/) plugin for [Scala.js](https://www.scala-js.org/).
 
-Work in progress.
+## Usage
+
+We assume that you have an existing Vite and Scala.js sbt project.
+If not, [follow the accompanying tutorial](https://github.com/scala-js/scala-js-website/pull/590).
+
+Install the plugin as a development dependency:
+
+```shell
+$ npm install -D @scala-js/vite-plugin-scalajs
+```
+
+Tell Vite to use the plugin in `vite.config.js`:
+
+```javascript
+import { defineConfig } from "vite";
+import scalaJSPlugin from "@scala-js/vite-plugin-scalajs";
+
+export default defineConfig({
+  plugins: [scalaJSPlugin()],
+});
+```
+
+Finally, import the Scala.js output from a `.js` or `.ts` file with
+
+```javascript
+import 'scalajs:main.js';
+```
+
+which will execute the main method of the Scala.js application.
+
+The sbt project must at least be configured to use ES modules.
+For the best feedback loop with Vite, we recommend to emit small modules for application code.
+If your application lives in the `my.app` package, configure the sbt project with the following settings:
+
+```scala
+scalaJSLinkerConfig ~= {
+  _.withModuleKind(ModuleKind.ESModule)
+    .withModuleSplitStyle(
+      ModuleSplitStyle.SmallModulesFor(List("my.app")))
+},
+```
+
+## Configuration
+
+The plugin supports the following configuration options:
+
+```javascript
+export default defineConfig({
+  plugins: [
+    scalaJSPlugin({
+      // path to the directory containing the sbt build
+      // default: '.'
+      cwd: '.',
+
+      // sbt project ID from within the sbt build to get fast/fullLinkJS from
+      // default: the root project of the sbt build
+      projectID: 'client',
+
+      // URI prefix of imports that this plugin catches (without the trailing ':')
+      // default: 'scalajs' (so the plugin recognizes URIs starting with 'scalajs:')
+      uriPrefix: 'scalajs',
+    }),
+  ],
+});
+```
+
+## Importing `@JSExportTopLevel` Scala.js members
+
+`@JSExportTopLevel("foo")` members in the Scala.js code are exported from the modules that Scala.js generates.
+They can be imported in `.js` and `.ts` files with the usual JavaScript `import` syntax.
+
+For example, given the following Scala.js definition:
+
+```scala
+import scala.scalajs.js
+import scala.scalajs.js.annotation._
+
+@JSExportTopLevel("ScalaJSLib")
+class ScalaJSLib extends js.Object {
+  def square(x: Double): Double = x * x
+}
+```
+
+we can import and use it as
+
+```javascript
+import { ScalaJSLib } from 'scalajs:main.js';
+
+const lib = new ScalaJSLib();
+console.log(lib.square(5)); // 25
+```
+
+### Exports in other modules
+
+By default, `@JSExportTopLevel("Foo")` exports `Foo` from the `main` module, which is why we import from `scalajs:main.js`.
+We can also split the Scala.js exports into several modules.
+For example,
+
+```scala
+import scala.scalajs.js
+import scala.scalajs.js.annotation._
+
+@JSExportTopLevel("ScalaJSLib", "library")
+class ScalaJSLib extends js.Object {
+  def square(x: Double): Double = x * x
+}
+```
+
+can be imported with
+
+```javascript
+import { ScalaJSLib } from 'scalajs:library.js';
+```
+
+The Scala.js documentation contains [more information about module splitting](https://www.scala-js.org/doc/project/module.html).