add sbt-version-policy and declare a version scheme

fixes #434

Author: SethTisue

build.sbt

@@ -70,6 +70,8 @@ lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform
         if (scalaVersion.value.startsWith("2.13.") || isDotty.value) sharedSourceDir / "scala-2.13"
         else sharedSourceDir / "scala-2.11_2.12"
       },
+      versionScheme := Some("early-semver"),
+      versionPolicyIntention := Compatibility.BinaryCompatible,
     )
     .jvmSettings(
       Test / unmanagedSourceDirectories += (ThisBuild / baseDirectory).value / "compat/src/test/scala-jvm",
@@ -361,6 +363,7 @@ inThisBuild(
             List(s"""++${sys.env.get("TRAVIS_SCALA_VERSION").get}!"""),
             List(s"$projectPrefix/clean"),
             List(s"$testProjectPrefix/test"),
+            List(s"$projectPrefix/versionPolicyCheck"),
             List(s"$projectPrefix/publishLocal"),
             publishTask
           ).flatten

project/plugins.sbt

@@ -12,3 +12,4 @@ addSbtPlugin("org.portable-scala"     % "sbt-scala-native-crossproject" % crossV
 addSbtPlugin("org.scala-lang.modules" % "sbt-scala-module"              % "2.2.4")
 addSbtPlugin("ch.epfl.scala"          % "sbt-scalafix"                  % "0.9.26")
 addSbtPlugin("com.eed3si9n"           % "sbt-buildinfo"                 % "0.10.0")
+addSbtPlugin("ch.epfl.scala"          % "sbt-version-policy"            % "1.0.0-RC5")