documentation of image (img:) support is missing

[kirchsth]

PR #173 added image support to the sprites (via img: prefix). Related documentation in readme.md is missing.

@startuml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml

AddRelTag("plantuml", $textColor="$ARROW_COLOR", $lineColor="$ARROW_COLOR", $sprite="img:http://plantuml.com/logo3.png{scale=0.3}", $legendSprite="img:http://plantuml.com/logo3.png{scale=0.1}", $legendText="console triggered")

Container(containerAlias, "Label", "Technology")
System(systemAlias, "Label", $sprite="img:http://plantuml.com/logo3.png")

Rel(containerAlias, systemAlias, "Label", $tags="plantuml")

SHOW_LEGEND()
@enduml