Mark cbt as experimental

Author: smarter

docs/docs/usage/cbt-projects.md

@@ -3,6 +3,9 @@ layout: doc-page
 title: "Using Dotty with cbt"
 ---
 
+**NOTE: cbt support for Dotty is experimental and incomplete (for example,
+incremental compilation is not supported), we recommend [using Dotty with sbt](sbt-projects.md) for now.**
+
 cbt comes with built-in Dotty support. Follow the
 [cbt tutorial](https://github.com/cvogt/cbt/), then simply extend `Dotty` in the Build class.