Improve scala_config() documentation in README.md (#1494)

Author: kolotyluk

README.md

@@ -57,10 +57,14 @@ http_archive(
     url = "https://github.com/bazelbuild/rules_scala/archive/%s.zip" % rules_scala_version,
 )
 
+load("@io_bazel_rules_scala//:scala_config.bzl", "scala_config")
 # Stores Scala version and other configuration
 # 2.12 is a default version, other versions can be use by passing them explicitly:
 # scala_config(scala_version = "2.11.12")
-load("@io_bazel_rules_scala//:scala_config.bzl", "scala_config")
+# Scala 3 requires extras...
+#   3.2 should be supported on master. Please note that Scala artifacts for version (3.2.2) are not defined in
+#   Rules Scala, they need to be provided by your WORKSPACE. You can use external loader like
+#   https://github.com/bazelbuild/rules_jvm_external
 scala_config()
 
 load("@io_bazel_rules_scala//scala:scala.bzl", "rules_scala_setup", "rules_scala_toolchain_deps_repositories")