Your project should have a license associated with it.
A license clarifies how the project can be used, what your rights and obligations are, etc. Not having a license creates ambiguity for your users, and may prevent them from using your project out of fear of failing to comply with rules and subsequent litigation.
The process of choosing and applying a license is out of the scope of this document. Here are a couple of resources that may help for open source cases:
The licensing information should be present in the project's root directory and referenced from the documentation entrypoint.