Add unit test dependency install instructions

MatroskaLib/MatroskaLib.Test/README.md

@@ -0,0 +1,20 @@
+# Unit tests
+
+## Setup
+The unit tests depend on ffmpeg and mkvalidator to make sure the program outputs valid MKV files. If you want to run the tests locally, please follow the steps below:
+
+**Windows**  
+- `winget install ffmpeg`
+- *mkvalidator.exe is already included for windows, no need to install*
+
+**Linux**
+- Install ffmpeg using your package manager or see the [ffmpeg website](https://ffmpeg.org/download.html) for instructions to install.
+- If you are on Arch Linux: [install mkvalidator from the AUR](https://aur.archlinux.org/packages/mkvalidator)  
+  If you are on a Ubuntu based distro: [use this PPA](https://launchpad.net/~hizo/+archive/ubuntu/mkv-extractor-gui) to install mkvalidator
+  Otherwise: [Download the source code](https://sourceforge.net/projects/matroska/files/mkvalidator/) and compile using make.
+
+**MacOS**  
+Use homebrew:
+```
+brew install mkvalidator ffmpeg
+```