Add import statement for using the library() class
Add import statement for using the library() class

When trying to use the library() class to import a shared library I kept getting an error saying the class was not available.  I figured it was in the `import com.lesfurets.jenkins.unit.*` statement, but after looking at the example [here](https://github.com/jenkinsci/JenkinsPipelineUnit/blob/master/src/test/groovy/com/lesfurets/jenkins/TestSharedLibraryWithProjectSourceRetriever.groovy) I noticed that it was in a separate path and needed to be imported as well.

I noticed under the SourceRetriever section it mentions you need to import those so I thought adding the import for the library() class would also be helpful to have in the readme.

Thanks.
lesfurets/JenkinsPipelineUnit