docs(export): add description and examples 📝

[ci skip]

Author: DrSensor

README.md

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+## Options
+
+### `export`
+How wasm code would be exported. (see [examples](#examples))
+- Type: `string`
+- Default: `async`
+- Expected value:
+  - `buffer` will export wasm code as [Buffer][]
+  - `module` will export wasm code as [WebAssembly.Module][]
+  - `instance` will export wasm code as [WebAssembly.Instance][]
+  - `async` will [instantiate][webassembly.instantiate] wasm code asynchronously, return promise of both [WebAssembly.Module][] and [WebAssembly.Instance][]
+  - `async-module` will [compile][webassembly.compile] wasm code asynchronously, return promise of [WebAssembly.Module][]
+  - `async-instance` will [instantiate][webassembly.instantiate] wasm code asynchronously, return promise of [WebAssembly.Instance][]
+
+<details><summary><i>webpack.config.js</i></summary>
+
+```js
+module.exports = {
+    rules: [{
+          test: /\.wasm$/,
+          type: "javascript/auto",
+          use: [{
+              loader: "webassembly-loader",
+              options: {
+                  export: "async"
+              }
+          }]
+    }]
+}
+```
+</details>
+
+> tips: you can use [query parameter][inline] to change export mode on demand
+
+[inline]: https://webpack.js.org/concepts/loaders/#inline
+[buffer]: https://nodejs.org/api/buffer.html
+[webassembly.module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Module
+[webassembly.instance]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance
+[webassembly.instantiate]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/instantiate
+[webassembly.compile]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/compile
+
+## Examples
+
+See the test cases and example projects in [*.test.ts](./test) and [examples](./examples/) for more insight.
+
+### With options
+
+#### `{export: 'buffer'}`
+
+```js
+import wasmCode from "./lib.wasm";
+
+WebAssembly.compile(wasmCode).then(module => {
+  const instance = new WebAssembly.Instance(module);
+  console(instance.exports.add(1, 2)); // 3
+});
+```
+
+---
+
+#### `{export: 'module'}`
+
+```js
+import wasmModule from "./lib.wasm";
+
+const instance = new WebAssembly.Instance(wasmModule);
+console(instance.exports.add(1, 2)); // 3
+```
+
+---
+
+#### `{export: 'instance'}`
+
+```js
+import wasm from "./lib.wasm";
+
+console(wasm.exports.add(1, 2)); // 3
+```
+
+---
+
+#### `{export: 'async'}`
+
+```js
+import wasmInstantiate from "./lib.wasm";
+
+wasmInstantiate(importObject | undefined).then(({ instance, module }) => {
+  console(instance.exports.add(1, 2)); // 3
+
+  // create different instance, extra will be called in different environment
+  const differentInstance = new WebAssembly.Instance(module);
+  console(differentInstance.exports.add(1, 2)); // 6
+});
+```
+
+---
+
+#### `{export: 'async-instance'}`
+
+```js
+import wasmInstantiate from "./lib.wasm";
+
+wasmInstantiate(importObject | undefined).then(instance => {
+  console(instance.exports.add(1, 2)); // 3
+});
+```
+
+---
+
+#### `{export: 'async-module'}`
+
+```js
+import wasmInstantiate from "./lib.wasm";
+
+wasmCompile(importObject | undefined).then(module => {
+  const differentInstance = new WebAssembly.Instance(module);
+  console(differentInstance.exports.add(1, 2)); // 3
+});
+```
+---