M README.md → README.md

@@ -6,12 +6,24 @@ Replace `www.youtube.com` or `youtu.be` with `y.outube.duckdns.org` to fix embeds for short videos.
 
 https://github.com/birabittoh/FixYouTube-legacy/assets/26506860/e1ad5397-41c8-4073-9b3e-598c66241255
 
+### Advanced usage
+Some services require video previews to be smaller than a certain file size. This app gets the best-looking format by default, so it's not uncommon to have previews that are too big, thus being ignored by crawlers.
+
+Enter the `/{videoID}/{formatID}` endpoint.
+
+You can change `formatID` to cycle through the available formats for a given video. These formats are also filtered based on file size, in a way that makes them viable Telegram previews.
+
+In short, if `/{videoID}` does not generate a preview, you can try `/{videoID}/1`, which should be the best-looking format within Telegram's filesize bounds.
+
+The next values of `formatID` (2 to infinity) are even smaller formats within those bounds.
+
+If the video is too long, there might not be small enough formats. In that case, the app returns error 500.
+
 ## Instructions
 
-First of all, you should set the API key to whatever you want:
+First of all, you should create a `.env` file:
 ```
 cp .env.example .env
-nano .env
 ```
 
 ### Docker with reverse proxy