what should be in the git description file?
description file is only used by the GitWeb program ( to display the description of the repo on the GitWeb page)
https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain
Otherwise, how you use it is left to you and I don't see a point in a "style" for it.
The description file can also be read by hook scripts or other scripts pertaining to the repository. For example, one popular hook script, post-receive-email, sends an email to all members when a commit is made to the repository, and uses the contents of the description file as the name of the repository in the email subject.
description is just a text file that is shown as project description in web frontend.
Just write there something that explains what the repository is about.