add query guide

guides/query.md

@@ -3,4 +3,40 @@ title: query
 subtitle: use the query function to parse query parameters
 ---
 
-## Coming Soon
+## Introduction
+
+The query function is just a helper function to help extract query parameters from URLs. It uses the WHATWG URL API under the hood to extract the query parameters and convert it from a [URLSearchParams](https://nodejs.org/api/url.html#url_class_urlsearchparams) to a JSON object.
+
+If there are multiple query parameters with the same name, it will return an array instead of a string.
+
+## Usage
+
+Simply import the `query` function and pass it the `req` object.
+
+```javascript
+const { light, query } = require('light');
+
+module.exports = light({
+  path: '/',
+
+  async handler(req) {
+    const { id, name } = query(req);
+
+    return {
+      id,
+      name,
+    };
+  },
+});
+```
+
+After starting the dev server, you can make a request to [localhost:3000/?id=123&name=light](http://localhost:3000/?id=123&name=light) and expect a response of
+
+```json
+{
+  "id": "123",
+  "name": "light"
+}
+```
+
+Note that by default all query parameters are strings, and you will need to convert them as necessary.