add ffmpeg fix notes

Author: arch

docs/app/docs/user-guide/build.md

@@ -35,7 +35,7 @@ cd Python-Funscript-Editor
 conda env create -f environment_ubuntu.yaml
 ```
 
-On some distributions e.g. Ubuntu 20.04 LTS, the FFmpeg package does not contain the required `v360` filter. Therefore you can download a static linked FFmpeg package for this application with `bash download_ffmpeg.sh` from project root directory. (Perform this step if you are not sure if your FFmpeg version is sufficient!).
+On some distributions e.g. Ubuntu 20.04 LTS, the FFmpeg package does not contain the required `v360` filter. Therefore you can download a static linked FFmpeg package for this application with `bash download_ffmpeg.sh` from project root directory. (Perform this step if you are not sure if your FFmpeg version is sufficient!). The latest ffmpeg downloaded via `download_ffmpeg.sh` break current MTFG. Therefore you should copy the ffmpeg executable from `./assets` to `./funscript_editor/data` for now.
 
 Ubuntu user can alternatively use the `savoury1` ppa:
 
@@ -66,6 +66,8 @@ sudo pacman -Sy python-opencv python-pyqt5
 
 All other required python packages can be installed from pip.
 
+The latest ffmpeg break current MTFG. Therefore you should copy the ffmpeg executable from `./assets` to `./funscript_editor/data` for now!!
+
 ## Flatpak
 
 The repository contains an flatpak build recipe that can be used to build an flatpak app. You can build the flatpak with the `build_flatpak.sh` script in the project root directory. The build script generate the `PythonFunscriptEditor.flatpak` package that can be installed with `flatpak install --user PythonFunscriptEditor.flatpak` on the system.