docs(gatsby-source-wordpress): add migration from experimental instru…

…ctions (#29401)

Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>

packages/gatsby-source-wordpress/docs/migrating-from-other-wp-source-plugins.md

@@ -14,6 +14,14 @@ Since `v3` used the WP REST API and `v4` uses WPGraphQL, the data shape and avai
 
 If you have any custom code which you've added to make connections between nodes work in `v3`, you will get to delete that code (yay!) because WPGraphQL has excellent support built in for connection fields between nodes (for example `Page.author` or `User.pages`).
 
+## Migrating from `gatsby-source-wordpress-experimental`
+
+This is the easiest migration since both plugins are one and the same (as of `gatsby-source-wordpress@4.0.0`).
+
+- Uninstall the experimental plugin `npm uninstall gatsby-source-wordpress-experimental`
+- Install the stable plugin `npm install gatsby-source-wordpress`
+- Then update `gatsby-config.js` to remove `-experimental` from the plugin name.
+
 ## Migrating from `gatsby-source-graphql`
 
 You're in luck! :four_leaf_clover: This will likely be a very easy migration!