Include "postcss-lit" package to the docs as a recommended solution to process css

Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Other/unknown (please mention in description)

Description

There is a package, which provides ability to process css inside lit components. For now, it's not mentioned anywhere in the repo as a solution which should be used in case if someone wants to integrate some tool like stylelint (although, it's mentioned in that repo).

In postcss-lit this discussion was also initiated since for now it's only one open source package with well defined docs which helps to do this (actually, there was a proposal to include that package to lit project and support it accordingly).

Alternatives and Workarounds

In case if we don't use this package, need to implement own custom solutions to somehow integrate the tools like stylelint in the project or, moreover, implement own lint tools for styles.

There is also one article prepared by creator of postcss-lit, how to adopt stylelint-processor-styled-components for lit components.

Start by locating the Lit documentation section covering CSS processing or stylelint integrations. Add postcss-lit as a recommended solution, linking to its repository and explaining that it processes CSS inside Lit components; confirm the documentation clearly addresses the use case described in the issue.