feat(plugins): Adds Mailchimp newsletter subscription

packages/medusa-plugin-mailchimp/README.md

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+# medusa-plugin-mailchimp
+
+Mailchimp plugin for Medusa that supports newsletter subscriptions.
+
+## Plugin Options
+
+```
+{
+  api_key: [your mailchimp api key] (required),
+  newsletter_list_id: ["123456789"] (required)
+}
+```
+
+## Dynamic usage
+
+You can resolve the Mailchimp service, such that your server can handle newsletter subscriptions.
+
+Example:
+
+```js
+const mailchimpService = req.scope.resolve("mailchimpService")
+mailchimpService.subscribeNewsletter(
+  "acme@mail.com",
+  // merge_fields to include in the subscription
+  { FIRSTNAME: "Acme", LASTNAME: "Inc." }
+)
+```