Gradle - Add single file to JAR

I have the file "license.txt" in the root directory of my project. In the jar-task, I want to add this file to the (root folder of the) JAR file.

I tried

jar {
  from '.' include 'license.txt'
}

but this replaces the other content (.class files) instead of adding a file. And I do not want to add the license.txt to the resources folder, because I do not want to change my project structure just because of the build tool.

Who can help? Thank you!