Add README

README.md

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+# http4s-scala-xml
+
+Provides http4s entity codec instances for [scala-xml].
+
+## `http4s-scala-xml`
+
+This is probably the artifact you want.  It works with scala-xml-2.x.
+
+### SBT coordinates
+
+```scala
+libraryDependencies ++= Seq(
+  "org.http4s" %% "http4s-scala-xml" % http4sScalaXmlV
+)
+```
+
+## `http4s-scala-xml-1`
+
+This repository also publishes an alternate `http4s-scala-xml-1`
+artifact.  The Scala package is the same, so this dependency must
+never be bundled with `http4s-scala-xml`.  It exists because several
+signficant libraries, like [Twirl], are still based on scala-xml-1.x
+in Scala 2.  Use this library to avoid diamond dependencies, but
+upgrade when you can.
+
+### SBT coordinates
+
+```scala
+libraryDependencies ++= Seq(
+  "org.http4s" %% "http4s-scala-xml-1" % http4sScalaXmlV
+)
+```
+
+
+## Compatibility
+
+| artifact           | version | http4s-core | scala-xml | Scala 2.12 | Scala 2.13 | Scala 3 | Status |   |
+|:-------------------|:--------|:------------|:----------|------------|------------|---------|--------|---|
+| http4s-scala-xml   | 0.23.x  | 0.23.x      | 2.x       | ✅         | ✅         | ✅      | Stable |   |
+| http4s-scala-xml-1 | 0.23.x  | 0.23.x      | 1.x       | ✅         | ✅         | ❌      | Stable |   |
+
+[scala-xml]: https://github.com/scala/scala-xml
+[twirl]: https://github.com/playframework/twirl