Add Instructions for Distrobox

[danyeaw]

Hi, thanks for the SDK, this definitely makes setting up a development environment much easier!

As a distrobox user, I did something like:

distrobox create --image ghcr.io/wkdev:latest wkdev
distrobox enter wkdev
git clone https://github.com/WebKit/WebKit.git WebKit
cd WebKit
Tools/Scripts/build-webkit --gtk

Would you be open to adding distrobox instructions?

[TingPing]

While the image alone is very useful, we also have some custom scripts that are expected to be ran.

Maybe we could document that, someone will need to test and write it all down.

[nikolaszimmermann]

What came to your mind @TingPing?
@danyeaw - did you have any issues building + using WebKit this way?