It's common on README.md to include badges or shields. However there isn't a default option for nimble but since nim uses repository tags for versioning it's possible to use a custom tag along with the included nim logo.
I've started using a tag for any projects I've added to nim-lang/packages in case others are interested.
It uses the github/v/tag API of shields.io.
The syntax for the badge is: /github/v/tag/:user/:repo?filter=v*&logo=Nim&label=nimble&labelColor=black&color=%23f3d400.
I am aware of this one used by some projects. But I prefer to use a consistent style for README badges.