Added a not on media inlining perf improvement

refs https://github.com/TryGhost/Toolbox/issues/524

- Fetching media from a remote server is an expensive network operation. Given there's probability for the content to reuse the same image in different posts or in multiple places, we could save on extra fetches by adding caching to the remote media fetch method

ghost/external-media-inliner/lib/ExternalMediaInliner.js

@@ -40,6 +40,8 @@ class ExternalMediaInliner {
      * @returns {Promise<Object>}
      */
     async #getRemoteMedia(requestURL) {
+        // @NOTE: this is the most expensive operation in the whole inlining process
+        //        we should consider caching the results to improve performance
         try {
             return await request(requestURL, {
                 followRedirect: true,