added docs

Author: corymharper

website/docs/functions/validate-url.md

@@ -2,4 +2,57 @@
 title: validateURL()
 ---
 
-Interested in this capability? Help us bring it into existence by submitting a PR! https://github.com/sparksuite/w3c-css-validator/issues/31
+This function is used to validate external CSS via a URL.
+
+##  Options
+
+You can customize the behavior with options, passed as the second argument.
+
+Option | Default | Possible values
+:--- | :--- | :---
+`medium` | `all` | `all`, `braille`, `embossed`, `handheld`, `print`, `projection`, `screen`, `speech`, `tty`, `tv`
+`warningLevel` | `0` | `0`, `1`, `2`, `3`
+`timeout` | `10000` | `integer`
+
+Option | Explanation
+:--- | :---
+`medium` | The equivalent of the `@media` rule, applied to all of the CSS
+`warningLevel` | `0` means don’t return any warnings; `1`, `2`, `3` will return warnings (if any), with higher numbers corresponding to more warnings
+`timeout` | The time in milliseconds after which the request to the W3C API will be terminated and an error will be thrown
+
+```ts
+const result = await cssValidator.validateURL(cssSourceURL, {
+    medium: 'print',
+    warningLevel: 3,
+    timeout: 3000,
+});
+```
+
+## Response structure
+
+By default, the function returns a Promise, which resolves to an object that looks like:
+
+```ts
+{
+    valid: boolean;
+    errors: {
+        line: number;
+        message: string;
+        url: string | null;
+    }[];
+}
+```
+
+If you ask it to return warnings via `warningLevel`, it will also include a `warnings` key:
+
+```ts
+{
+    ...
+    warnings: {
+        line: number;
+        level: 1 | 2 | 3;
+        message: string;
+        url: string | null;
+    }[];
+}
+```