Add import for defaultShellScript to README
Add import for defaultShellScript to README

Resolves #153

On SBT 0.13.7 I get this error without the import
```
assemblyOption in assembly := (assemblyOption in assembly).value.copy(prependShellScript = Some(defaultShellScript))
                                                                                                ^
[error] Type error in expression
```
onilton/sbt-assembly,sbt/sbt-assembly,sbt/sbt-assembly