Add README.md for xilem. (#268)

This is a basic skeleton based on the latest Linebender standards.

I also added a paragraph about the reactive layer's origins.
Plus a section describing the modularity of our stack.

Additional sections describing more of the actual new state of things
are welcome in follow-up PRs, but are not a goal of this PR.

crates/xilem_masonry/README.md

@@ -0,0 +1,58 @@
+<div align="center">
+
+# Xilem
+
+**An experimental Rust architecture for reactive UI**
+
+[![Latest published version.](https://img.shields.io/crates/v/xilem.svg)](https://crates.io/crates/xilem)
+[![Documentation build status.](https://img.shields.io/docsrs/xilem.svg)](https://docs.rs/xilem)
+[![Apache 2.0 license.](https://img.shields.io/badge/license-Apache--2.0-blue.svg)](#license)
+
+[![Linebender Zulip chat.](https://img.shields.io/badge/Linebender-%23xilem-blue?logo=Zulip)](https://xi.zulipchat.com/#narrow/stream/354396-xilem)
+[![GitHub Actions CI status.](https://img.shields.io/github/actions/workflow/status/linebender/xilem/ci.yml?logo=github&label=CI)](https://github.com/linebender/xilem/actions)
+[![Dependency staleness status.](https://deps.rs/crate/xilem/latest/status.svg)](https://deps.rs/crate/xilem)
+
+</div>
+
+Xilem is a UI toolkit.
+It combines ideas from Flutter, SwiftUI, and Elm.
+Like all of these, it uses lightweight view objects, diffing them to provide minimal updates to a retained UI.
+Like SwiftUI, it is strongly typed.
+For more details on Xilem's reactive architecture see [Xilem: an architecture for UI in Rust].
+
+Xilem's reactive layer is built on top of a wide array of foundational Rust UI projects, e.g.:
+* Widgets are provided by [Masonry], which is a fork of the now discontinued [Druid] UI toolkit.
+* Rendering is provided by [Vello], a high performance GPU compute-centric 2D renderer.
+* GPU compute infrastructure is provided by [wgpu].
+* Text support is provided by [Parley], [Fontique], [Swash], and [Skrifa]. 
+* Accessibility is provided by [AccessKit].
+* Window handling is provided by [winit].
+
+Xilem can currently be considered to be in an alpha state.
+Lots of things need improvements.
+
+## Community
+
+Discussion of Xilem development happens in the [Linebender Zulip](https://xi.zulipchat.com/), specifically the [#xilem stream](https://xi.zulipchat.com/#narrow/stream/354396-xilem).
+All public content can be read without logging in.
+
+Contributions are welcome by pull request. The [Rust code of conduct] applies.
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache 2.0 license, shall be licensed as noted in the [License](#license) section, without any additional terms or conditions.
+
+## License
+
+Licensed under the Apache License, Version 2.0 ([LICENSE](LICENSE) or <http://www.apache.org/licenses/LICENSE-2.0>)
+
+[Masonry]: https://crates.io/crates/masonry
+[Druid]: https://crates.io/crates/druid
+[Vello]: https://crates.io/crates/vello
+[wgpu]: https://crates.io/crates/wgpu
+[Parley]: https://crates.io/crates/parley
+[Fontique]: https://crates.io/crates/fontique
+[Swash]: https://crates.io/crates/swash
+[Skrifa]: https://crates.io/crates/skrifa
+[AccessKit]: https://crates.io/crates/accesskit
+[winit]: https://crates.io/crates/winit
+[Xilem: an architecture for UI in Rust]: https://raphlinus.github.io/rust/gui/2022/05/07/ui-architecture.html
+[Rust code of conduct]: https://www.rust-lang.org/policies/code-of-conduct