I've noticed that some of the modules' summaries can not be seen in the listing on the backdropcms.org. I assume there is a requirement for a number of symbols for the first paragraph in modules Readme.md that is used then as a brief info about the module. Do you know the requirement for shaping up the readme or where can I read about them?

There is a module template at https://github.com/backdrop-contrib/module_template/blob/main/README.md?plain=1, including the following information about the first README paragraph: