Read “The Role of Formalism in System Requirements”. It is a pretty general paper on requirements (requirement documents/specifications) and the languages you can use to write them (from natural language to formal languages). I got a great link about i* though:
https://github.com/jhcp/piStar. It is a i* tool, supporting the i* 2.0 specification. It is easy to install and run it: simply download the files (clone the repository) and load the index.html. That is, quite simple! Another cool site I've found is
https://www.capterra.com/requirements-management-software/, a directory on requirements engineering tools. I used that directory to update my course with links to several tools (and tutorials).