Enhanced README.md to include instruction to build and publish for Scala 2.13.

Author: cheeseng

README.md

@@ -120,6 +120,6 @@ To publish scalactic, scalatest and scalatest-app (for Scala and Scala-js, versi
 
   `$ sbt clean publishSigned "project scalatestAppJS" clean publishSigned ++2.10.6 "project scalatestApp" clean publishSigned "project scalatestAppJS" clean publishSigned`
 
-To publish scalactic, scalatest and scalatest-app (for Scala and Scala-js, version 2.12, and make sure you're on Java 8) to Sonatype, use the following command:
+To publish scalactic, scalatest and scalatest-app (for Scala and Scala-js, version 2.12 and 2.13, and make sure you're on Java 8) to Sonatype, use the following command:
 
-  `$ sbt ++2.12.0 clean publishSigned "project scalatestAppJS" clean publishSigned`
+  `$ sbt ++2.12.0 clean publishSigned "project scalatestAppJS" clean publishSigned ++2.13.0-M2 clean publishSigned "project scalatestAppJS" clean publishSigned`