Add docker image to readme

[rossmcdonald]

We have a public Docker image available for Spectral available on Docker hub as:

stoplight/spectral

We might want to add that to the installation section with a little snippet, like:

docker run --rm -it stoplight/spectral lint "${URL}"

[stoplight-bot]

🎉 This issue has been resolved in version 2.2.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀