Add documentation for changing base font-size

This may be intuitive enough to some, but it took me a minute to figure out so I think it'd be nice to have here.

docs/src/pages/configurations/add-custom-body/index.md

@@ -11,6 +11,16 @@ You can accomplish this by creating a file called `preview-body.html` inside the
 <div id="custom-root"></div>
 ```
 
+If using relative sizing in your project (like `rems` or `ems`), you may update the base `font-size` by adding a `style` tag to `preview-body.html`:
+
+```html
+<style>
+  body {
+    font-size: 15px;
+  }
+</style>
+```
+
 That's it. Storybook will inject these tags to html body.
 
 > **Important**