Clarify `.face.icon` format requirements in documentation

[noirith]

Possible Clarification in Documentation: .face.icon File Format

Hi! First of all, thanks for the amazing project and all the effort behind it.

I recently ran into a small issue that I believe might affect other users, especially newcomers or those transitioning from Windows systems.

The documentation mentions the use of ~/.face.icon as a fallback avatar when MPRIS is not available. However, it's easy to assume that this file should be an actual .ico Windows icon file, while in practice, it must be a plain .png or .jpg image with the file named .face.icon.

In my case, I accidentally used a real .ico file downloaded from the internet, and even though the file was named .face.icon, my widget could not render the image. After converting the file to a proper .png and keeping the .face.icon name, it worked as expected.

Suggestion:

It might be worth clarifying this in the documentation or README, stating that:

• The file ~/.face.icon must be a valid image, ideally .png or .jpg.

• The name .face.icon is a naming convention, but the internal format cannot be .ico (Windows icon resource).

• Users can check the file format with:

  file ~/.face.icon
  

Just a small detail, but I believe it could prevent confusion for others.

Thanks again for the project and for your time!