Add useful twig variables to README

Problem/Motivation

When i was configuring my project to handle CM Notifications, i needed to provide a dynamic email sender.
To do so, i had to find the right twig variables to achieve my goal.

Proposed resolution

Add the table below into readme in order to help other users to easily find those most useful tokens

USEFUL TWIG VARIABLES
---------------------

The most commonly used Twig variables for notifications are:

Current User Email:                       {{ user.email }}
Current User Username (the login name):   {{ user.username }}
Current User DisplayName:                 {{ user.displayname }}
Current User ID (uid):                    {{ user.id }}

Author Email:                             {{ entity.Owner.mail.0.value }}
Author Username (the login name):         {{ entity.Owner.username }}
Author DisplayName:                       {{ entity.Owner.displayname }}
Author id (uid):                          {{ entity.Owner.id }}

Entity Title:                             {{ entity.title.value }}
Entity Bundle (Content type system name): {{ entity.bundle }}
Entity Bundle Label (Content type name):  {{ entity.type.entity.label }}
Entity ID:                                {{ entity.nid.value }}
Entity Revision:                          {{ entity.vid.value }}
Entity UUID:                              {{ entity.uuid.value }}