docs: Update bundling documentation with an example using Astro (#358)

packages/docs/content/docs/add-to-your-site-bundling.mdx

@@ -12,13 +12,19 @@ To get started you need to install Static CMS via a package manager and save it
 
 ```bash
 // npm
-npm install @staticcms/core
+npm install @staticcms/core@latest
 
 // yarn
-yarn add @staticcms/core
+yarn add @staticcms/core@latest
 ```
 
-Then import it (assuming your project has tooling for imports):
+Then create a new route for your project (for instance at `/admin`), and import Static CMS:
+
+```js
+import CMS from '@staticcms/core';
+```
+
+The default export is a _CMS_ object, which has an `init` method that takes an object with a `config` attribute. The `config` attribute is an object representing the [configuration options](docs/configuration-options). You can use destructuring assigment syntax as shorthand: 
 
 ```js
 import CMS from '@staticcms/core';
@@ -32,9 +38,24 @@ CMS.registerPreviewTemplate('my-template', MyTemplate);
 
 **Note**: Wherever you initialize Static CMS (via `CMS.init()`), it takes over the current page. Make sure you only run the initialization code on your CMS page.
 
+If the CMS object is initialized without being passed an object with a valid config attribute, it will try to fetch and read a `config.yml` file, via http, within the same path where the CMS resides (`/admin/config.yml`).
+
+```js
+import CMS from '@staticcms/core';
+
+// Initialize the CMS object
+CMS.init();
+// Now the registry is available via the CMS object.
+CMS.registerPreviewTemplate('my-template', MyTemplate);
+```
+
+**Note**: Because `config.yml` is requested via http, make sure `<siteurl>/admin/config.yml` exists as an endpoint on your build. If the file is not placed in the public folder, this might not be the default behaviour for your static site generator.
+
+Make sure the file containing the CMS object will be built as a page, with `@staticcms/core` bundled, and the code including `CMS.init()` being run inside a script tag. This is what might take some time, as it will be done differently based on your static site generator. Check your static site generators's documentation for further details. 
+
 ## Configuration
 
-Configuration is different for every site, so we'll break it down into parts. Add all the code snippets in this section to your `admin/config.js` file (which is passed into the `CMS.init({ config })` call). Alternatively, you can use a yaml file (`admin/config.yml`) instead of a javascript file.
+Configuration is different for every site, so we'll break it down into parts. Add all the code snippets in this section to your `admin/config.js` file (which is passed into the `CMS.init({ config })` call).
 
 ### Backend