Readme for Scala plugin added (#7286)

plugins/scala/README.md

+## Scala plugin
+
+Completion script for [scala and scalac](https://www.scala-lang.org/) commands.
+
+To use it, add `scala` to the plugins array of your zshrc file:
+```
+plugins=(... scala)
+```
+
+## Aliases
+
+| Command          | Description                                                                     |
+|------------------|---------------------------------------------------------------------------------|
+| `scala`          | Run code in the Scala language                            |
+| `scalac`         | Compiler for the Scala language                           |