While uploading your files, make sure you initialize your repository using README - you might have noticed this in the checkbox provided when you create your repo on Github:
If you have already uploaded your files and created repo without a README, choose the 'Create New file' button and give the file name as README.md
This page is what anyone who is visiting your github profile or your repository would see first. So use this page to explain what your project is and why you've created it.
Also, keep a heading 'References' and add links to any sites that helped you in building the project. Do mention if you have used any CSS snippets as well here.
You can treat the README.md file just as you would do any simple text editor.
While READMEs can be written in any text file format, the most common one that is used nowadays is Markdown. You can read more about how to make your text bold, italic, or create headings : https://commonmark.org/help/