add postcss-lit adoption rfc

rfcs/NNNN-postcss-lit-adoption.md

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+---
+Status: Active
+Champions: @43081j
+PR: #17
+---
+
+# Official postcss/tailwind support
+
+As part of unifying the standard tools around Lit, it would make sense to move
+the [postcss-lit](https://github.com/43081j/postcss-lit) project into the
+Lit monorepo as an official solution to inline CSS transforms (e.g. tailwind).
+
+## Objective
+
+A strategy for how we will move the project into the monorepo.
+
+### Goals
+- Moving the postcss-lit project into the Lit monorepo
+- Potentially documenting the project in an appropriate place (or a blog post)
+
+### Non-Goals
+- Transfer of responsibility (it is expected I will continue being the primary
+maintainer to avoid extra burden on the Lit team)
+
+## Motivation
+
+The project already functions well and has high usage, but could benefit hugely
+in terms of discovery and support by being in the monorepo.
+
+- **Discovery:** we are regularly asked by users how to integrate Lit with
+tailwind and other such CSS solutions. Having an officially supported and
+maintained integration will make this clearer
+- **Support:** we are more likely to receive community contributions through
+issues and pull requests
+
+It will also become much easier to keep the integration in sync with the
+most current version of Lit, as we are likely to update it in line with
+any breaking changes elsewhere in the monorepo.
+
+## Detailed Design
+
+A new `packages/postcss-lit` directory can be created in the monorepo which
+is initially an exact copy of the existing `postcss-lit` package.
+
+Once the sources have been copied across, we can continue publishing the package
+under the existing `postcss-lit` name for backwards compatibility.
+
+After the move has been made, we should also do the following:
+
+- Migrate the CI workflow to be better aligned with other pipelines in the
+monorepo
+- Add a new `examples/` directory with full examples of how and when to use
+`postcss-lit`
+  - In particular, we should include a tailwind example, and a basic example
+
+## Implementation Considerations
+
+### Implementation Plan
+
+A single PR should be enough to begin with: copying the existing sources
+to the agreed directory and ensuring CI is setup correctly.
+
+### Backward Compatibility
+
+N/A
+
+### Testing Plan
+
+Existing tests should be enough once copied across.
+
+### Performance and Code Size Impact
+
+N/A
+
+### Interoperability
+
+N/A
+
+### Security Impact
+
+N/A
+
+### Documentation Plan
+
+We should create articles explaining the two ways users can transform
+CSS:
+
+- Inline CSS via postcss-lit
+- External CSS via rollup plugins (using CSS imports)
+
+These could be linked in the Lit website.
+
+In addition to these articles, we should publish some working examples (as
+mentioned earlier) in the repo itself. Particularly tailwind, a basic setup,
+and possibly an example of using postcss transforms.
+
+## Downsides
+
+N/A
+
+## Alternatives
+
+N/A