Updated link to Windows ffmpeg builds

[phantomtypist]

The old link (and domain) no longer work. I changed the link to the the ffmpeg project's download page for Windows binaries. They seem to keep that page updated.

Guidelines for Contributing

Airsonic development is a community project, and contributions are welcomed. Here are a few guidelines you should follow before submitting:

1. Styling The documentation follows some styling guidelines to fit with the website.
   - Follow the markdown guidelines. For more details see this guide.
   - Do not use <h1> or <h2> headers ( # h1 or ## h2 headers).
   - Never manually start a new line in the middle of a sentence.
   - More spacing, more empty lines !! The documentation needs to be readable !
   - Read existing guides to find some inspiration.

2. Testing The documentation is browsed with the website, so we recommend to test any new guides within the website using jekyll locally.
   - Clone the website repo git clone https://github.com/airsonic/airsonic.github.io.
   - Change directory into the cloned repo cd airsonic.github.io.
   - Clone the documentation submodule git clone https://github.com/airsonic/airsonic-docs docs.
   - Install ruby with the devkit. If you run it on Windows, don't install it to C:\Program Files as you will get trouble with the space later!
   - Install bundler (gem install bundler).
   - Install jekyll (gem install jekyll).
   - Update local dependencies: bundler update.
   - Install local dependencies: bundler install.
   - Run bundler exec jekyll serve --watch.
   - Add your changes.
   - Check if everything is fine at localhost:4040.

3. License Acceptance All contributions must be licensed under GNU GPLv3 to be accepted. Use git commit --signoff to acknowledge this.

4. Be bold! Without contributions, this project will vanish.

5. Stay relevant Issues or commentary that is off-topic or tangential to Airsonic development is subject to moderation. Questions should be focused on improving documentation to solve a problem. Visit Reddit or IRC for community discussion.