[DOC] Add hero image in the README

[tbouffard]

closes #43

How to create the screenshot

Run the following R script (for instance, in RStudio)

bpmn_file <- system.file("examples/Email_Voting.bpmn", package = "bpmnVisualization")

overlays <- list(
	# before 1st subprocess
	bpmnVisualization::create_overlay("sequence_flow_3", "26"),
	bpmnVisualization::create_overlay("sequence_flow_4", "69"),
	# 1st subprocess
	bpmnVisualization::create_overlay("sub_process_1", "69"),
	bpmnVisualization::create_overlay("send_task_1", "69"),
	# branch 1
	bpmnVisualization::create_overlay("user_task_2", "69"),
	bpmnVisualization::create_overlay("sequence_flow_34", "69"),
	# branch 2
	bpmnVisualization::create_overlay("user_task_3", "69"),
	bpmnVisualization::create_overlay("sequence_flow_31", "30"),
	bpmnVisualization::create_overlay("sequence_flow_32", "39"),
	bpmnVisualization::create_overlay("sequence_flow_36", "69"),
	# branch 3
	bpmnVisualization::create_overlay("intermediate_catch_event_2", "69"),
	bpmnVisualization::create_overlay("send_task_2", "69"),
	bpmnVisualization::create_overlay("sequence_flow_28", "69"),
	# end of 1st subprocess
	bpmnVisualization::create_overlay("task_1", "69"),
	# after 1st subprocess
	bpmnVisualization::create_overlay("sequence_flow_5", "69")
)

bpmnVisualization::display(bpmn_file, overlays)

Prior doing the screenshot, play with the zoom level to not be affected by process-analytics/bpmn-visualization-js#994 and process-analytics/bpmn-visualization-js#1236