README
Unity-Extensions
Variant 1
- Select "Add package from git URL"
- Paste
https://github.com/IvanMurzak/Unity-Extensions.git#upm

Variant 2
Modify manifest.json file.
{
"dependencies": {
"extensions.unity.base": "https://github.com/IvanMurzak/Unity-Extensions.git#upm"
}
}