Update Readme with information for multiple spaces (#21740)

* Update Readme with information for multiple spaces

Added in an explanation for sourcing from multiple Contentful environments/spaces. This is a topic I have seen come up multiple times in the community so I thought it warranted adding it here.

* Update README.md

* Update packages/gatsby-source-contentful/README.md

Co-Authored-By: LB <laurie@gatsbyjs.com>

* Update packages/gatsby-source-contentful/README.md

Co-Authored-By: Khaled Garbaya <khaledgarbaya@gmail.com>

* chore: format

* Update README.md

Added ` to gatsby-config.js

Co-authored-by: LB <laurie@gatsbyjs.com>
Co-authored-by: Khaled Garbaya <khaledgarbaya@gmail.com>
Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>

packages/gatsby-source-contentful/README.md

@@ -392,5 +392,31 @@ documentToReactComponents(node.bodyRichText.json, options)
 
 Check out the examples at [@contentful/rich-text-react-renderer](https://github.com/contentful/rich-text/tree/master/packages/rich-text-react-renderer).
 
+## Sourcing From Multiple Contentful Spaces
+
+To source from multiple Contentful environments/spaces, add another configuration for `gatsby-source-contentful` in `gatsby-config.js`:
+
+```
+// In your gatsby-config.js
+module.exports = {
+  plugins: [
+    {
+      resolve: `gatsby-source-contentful`,
+      options: {
+        spaceId: `your_space_id`,
+        accessToken: process.env.CONTENTFUL_ACCESS_TOKEN,
+      },
+    },
+    {
+      resolve: `gatsby-source-contentful`,
+      options: {
+        spaceId: `your_second_space_id`,
+        accessToken: process.env.SECONDARY_CONTENTFUL_ACCESS_TOKEN,
+      },
+    }
+  ],
+}
+```
+
 [dotenv]: https://github.com/motdotla/dotenv
 [envvars]: https://gatsby.dev/env-vars