Update README.md

Change the title

Add functionality to the slidebar

Add a slider and modify general design

Change buttons shadows

Change the shadows of the elements

Refactoring code for legibilty

Add a footer with copyright information

Add space at the bottom of the title and change the font-family of the buttons

Adjust the space between elements in the page

Add style to the buttons and canvas

Add functionality to the rainbow button

Fix a bug that does not allow to trigger the mouseup event, when the mouse was not inside the canvas

Increase the canvas size

Add functionability to the eraser button