The easiest way to add a module to a git repo, while still being able to develop the module as a standalone library, is with git subtree.
To do this, first set up the sub repo as a remote (let's call the remote foo):
git remote add foo https://github.com/hello/world.git
Next, add the sub repo as a subtree into the hello-world directory (which will be created automatically) in the root of the main repo:
git subtree add --prefix hello-world foo master --squash
That's it! If you don't need to sync with foo upstream, you don't need to do anything else.
To pull in new changes from foo, run:
git subtree pull --prefix=hello-world foo master --squash
To push new changes to foo, run:
git subtree push --prefix=hello-world foo master