Add temporary Scala 3 readme

readme-scala-3.md

@@ -0,0 +1,20 @@
+## WIP Scala 3 support via Scala 2.13 / 3 compatibility
+
+Relies on the `compiler` module introduced in https://github.com/lihaoyi/Ammonite/pull/1144.
+
+Run with
+```text
+$ ./mill -i 'amm[3].run'
+```
+
+Only Scala `3.0.0-M1` works for now, as it's the only Scala 3 version that can both depend on and
+be depended upon by Scala `2.13.4` modules. (The [Tasty reader version](https://github.com/lampepfl/dotty/blob/353677355f3a5ffe38b9abb31f07e61fbb37f50b/tasty/src/dotty/tools/tasty/TastyFormat.scala#L266-L267)
+is the same for both `2.13.4` and `3.0.0-M1`. It was incremented in `3.0.0-M2` and `3.0.0-M3`, which
+prevents `2.13.4` to read their Tasty files.)
+
+pprint wasn't published for `3.0.0-M1`, so we use the `2.13` version. We can't use the
+`TPrint` macros from Scala 3 because of that, so types are printed as `???`. Simple
+unit tests (like `./mill 'amm.repl[3].test' ammonite.session.AdvancedTests.pprint`) pass,
+thanks to a regex replacing types in the test fixtures with `???`. Not sure how far we
+can get with that approach. [Scala version decoupling](https://github.com/alexarchambault/Ammonite/tree/wip-scala-3)
+doesn't suffer that tasty reader version problem, so we can use `3.0.0-M2` fine for it.