Install appstream-doc by entering the following commands in the terminal:
sudo apt update sudo apt install appstream-doc
Developer documentation for AppStream
AppStream is a metadata specification, allowing to write metadata for software projects in a standardized format. This allows software centers to display useful information for not installed software, as well as determining software the user might want to install (e.g. to open an unknown file type or to make missing firmware available). . This package provides developer documentation for AppStream.