added readme links

packages/medusa-plugin-contentful/README.md

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+# medusa-payment-contentful
+
+Integrate Contentful with Medusa for rich CMS functionalities.
+
+Learn more about how you can use this plugin in the [documentaion](https://docs.medusajs.com/add-plugins/contentful).
+
+## Options
+
+```js
+{
+  space_id: CONTENTFUL_SPACE_ID,
+  access_token: CONTENTFUL_ACCESS_TOKEN,
+  environment: CONTENTFUL_ENV,
+}
+```