Update the documentation about webp

[biletboh]

For some developers, it might be obvious that you need to have libwebp package installed to add web support. Anyway, I think it would be helpful to add couple of lines to documentation about this.

For example:

"Make sure that a relevant libwebp library is installed to enable webp support."

[jrief]

Normally, this third party dependency is pulled in by Pillow anyway. In my installations, I usually rely on Debian Buster. There all image formats are supported out of the box. Which operating system are you using?

[biletboh]

I use docker based on alpine linux. Alpine is a very lightweight distribution, so it might lack some of the Debian defaults.

[jrief]

If you would use the official Python base image from DockerHub (which I use and is based on Buster), this should fix your problems.

[biletboh]

Thank you, I will consider using official python image.