message: Add graph of controller code to controller/README.md.
  Mostly I just want to check this in somewhere because I've included this
  graph in our covent submission.
  GPC: controller-graph
diff: Here resides the code for the ventilator controller.
  +## High-level graph of components
  +
  +<!-- See https://github.com/TLmaK0/gravizo -->
  +![Graph of components in cycle controller](https://g.gravizo.com/source/cc_graph_mark?https%3A%2F%2Fraw.githubusercontent.com%2FRespiraWorks%2FVentilatorSoftware%2Fmaster%2Fcontroller%2FREADME.md)
  +<!--
  +cc_graph_mark
  +digraph G {
  + subgraph cluster_RPI {
  + label="GUI (Raspberry Pi)"
  + labelloc=b;
  + GUI [label="Qt app"]
  + }
  +
  + subgraph cluster_CC {
  + label = "Cycle Controller (STM32)";
  + labelloc = b;
  + "Main loop" -> "Controller"
  + "Main loop" -> "Comms"
  +
  + "Controller" -> "Sensors"
  + "Controller" -> "Actuators"
  + "Controller" -> "Breath FSM"
  +
  + "Actuators" -> HAL
  + "Sensors" -> HAL
  + }
  +
  + "Comms" -> GUI
  + GUI -> "Comms"
  +}
  +cc_graph_mark
  +-->
  +
  ## Rationale and structure
  The ventilator actuators must be driven by control loops ensuring continuous
  