diff --git a/README.md b/README.md
index 5e128a3..99caafd 100644
--- a/README.md
+++ b/README.md
@@ -30,3 +30,18 @@ Now dashboard is available at `http://localhost:8000/`.
 ## other frontend
 
 - [Flutter frontend](https://github.com/lifegpc/eh_downloader_flutter)
+
+## FFI Extensions
+
+All dynamic libraries should place in `./lib` directory.
+
+### Thumbnail
+
+- Required tools: `cmake`, C/C++ compiler.
+- Required library: `libavformat`, `libavcodec`, `libavutil`, `libswscale`.
+
+#### Location
+
+- `./lib/thumbnail.dll` on Windows.
+- `./lib/libthumbnail.so` on Linux.
+- `./lib/libthumbnail.dylib` on macOS.