Update documentation for Scala 3

Author: pikinier20

README.md

@@ -48,6 +48,29 @@ is sticky.  To turn it back off when you're done running reports, use the
 `coverageOff` command or reset `coverageEnabled` with `set coverageEnabled :=
 false`.
 
+### Usage with Scala 3
+
+In Scala 3, the compiler itself takes care of the work that was done in Scala 2 by a compiler plugin. Unfortunately, currently the sbt plugin doesn't apply the neccessary options automatically.
+However, you can easily do it yourself, and use the plugin to generate user-friendly reports.
+
+1. Add the scoverage sbt plugin by appending this line to your `project/plugins.sbt`
+```scala
+addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.0.0-M4")
+```
+
+2. Compile your project with
+```scala
+Compile/compile/scalacOptions +=
+  s"-coverage-out:target/scala-${scalaVersion.value}/scoverage-data"
+```
+You can also set `-sourceroot:PATHS_ROOT` to customize how the path of your source files are resolved.
+Note that `-sourceroot` also sets the root path of the SemanticDB files.
+
+3. Run the tests: `sbt test`
+
+4. Generate xml and html reports: `sbt coverageReport`
+
+
 ### Multi project reports
 
 By default, scoverage will generate reports for each project separately. You can