docs(gatsby-source-shopify): Updates Shopify README with new plugin i…

…nfo (#31287)

Helps users use the latest Shopify source plugin (until it's merged into the monorepo)

packages/gatsby-source-shopify/README.md

@@ -1,4 +1,15 @@
-# gatsby-source-shopify
+# 📣 Looking for the latest version of `gatsby-source-shopify`?
+
+👉  Get started at [`gatsbyjs/gatsby-source-shopify`](https://github.com/gatsbyjs/gatsby-source-shopify)
+* Support for incremental builds
+* Scale up to 10k products
+* Up to 16x faster builds
+
+`gatsbyjs/gatsby-source-shopify` will replace the below package with the next minor release of `gatsbyjs/gatsby`.
+
+---
+
+## Legacy source plugin instructions:
 
 Source plugin for pulling data into [Gatsby][gatsby] from [Shopify][shopify]
 stores via the [Shopify Storefront API][shopify-storefront-api].