Help is based in man(1) pages. To supply help with your own plugin or command, create one or more man.1~7 pages and add them to your project under the corresponding man/man% directory.
Help for my_plugin is available via man(1). To add documentation to a fisher command, prepend the keyword fisher- to the man file, e.g., fisher-my-command.1. This will teach Fisherman how to access the man page using fisher help my-command.