Update README.md

I recently ran into an issue of using `sbt-assembly` inside a `Build.scala`
`assemblySettings` is defined as part of `sbtassembly.Plugin`, so if I don't have `import sbtassembly.Plugin._` inside `Build.scala`, the build will not compile. 

Maybe it's a good idea to add a note about how to set it up in a `Build.scala`?