The most basic piece of documentation is a Readme.
When creating a repository it is a good practise to add a Readme right from the start. Its the first point of touch with your repository.
A basic Readme should contain the following:
the repository’s purpose
an installation guide
a small code snippet of a typical use-case
a note on contribution
how you liscense your project (default choice: https://choosealicense.com/licenses/mit/)
I advice you to write documentation, because
essential if someone else will start using your software
you reflect your own design choices while explaining them to the user
in case of multiple users its the most efficient way to handle questions
great lookup for yourself