Unity - Add a description to a custom script

GameManager.cs

using UnityEngine;

public class GameManager : MonoBehaviour { }

GameManagerEditor.cs

using UnityEditor;

/**
 * Remember that you need to save this .cs inside any Editor folder
 */
[CustomEditor(typeof(GameManager))]
public class GameManagerEditor : Editor {
    public override void OnInspectorGUI() {
        EditorGUILayout.HelpBox(
            "Lorem ipsum dolor sit, amet consectetur adipisicing elit. Quos, explicabo",
            MessageType.Info
        );
        EditorGUILayout.Separator();
        base.OnInspectorGUI();
    }
}