Added a README.

README.md

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+# `ffmpeg-fetcher`
+This will download https://ffmpeg.zeranoe.com/builds/win64/shared/ffmpeg-latest-win64-shared.zip, extract it, and install it to `C:\opt\bin\ffmpeg`.
+
+## Steps
+- Create `C:\opt\` (Make a pull request if you want to add command-line options or something. It's fine for my uses and I don't care.)
+- Install 7-Zip if you don't have it already. Make sure it's the right arch because `ffmpeg-fetcher` looks for it at `C:\Program Files\7-Zip\7zG.exe`.
+- After install has finished, add `C:\opt\ffmpeg\bin` to your path (only need to do this part one time, obviously).