add usage guide

Author: Bims-sh

README.md

@@ -1,3 +1,13 @@
 # ghcss-extension
 
-A simple extension that allows users to view custom css on GitHub profiles.
+A simple extension that allows users to view custom css on GitHub profiles.
+
+# Usage
+### For User Pages
+Create a `user.css` file inside your user profile repository (e.g. https://github.com/someuser/someuser).
+
+### For Repositories
+Create a `repo.css` file inside your repository.
+
+### For Organizations
+Create a `org.css` file inside the `.github` repository (e.g. https://github.com/someorg/.github)