Readme: Describe adding a widget
Readme: Describe adding a widget

kasemir/org.csstudio.display.builder,ESSICS/org.csstudio.display.builder,kasemir/org.csstudio.display.builder,kasemir/org.csstudio.display.builder,ESSICS/org.csstudio.display.builder,ESSICS/org.csstudio.display.builder,kasemir/org.csstudio.display.builder,kasemir/org.csstudio.display.builder,ESSICS/org.csstudio.display.builder,ESSICS/org.csstudio.display.builder