Add Gitter badge.
Add Gitter badge.

Change-Id: Iaff00e4135ec86e4b7ad6675051688908d356d5a

shahrzadmn/vaadin-grid,manolo/components,manolo/components,jforge/components,shahrzadmn/vaadin-grid,shahrzadmn/vaadin-grid,jforge/components,manolo/components,shahrzadmn/vaadin-grid,jforge/components