Add pluggable discovery states documentation

Author: silvanocerza

docs/img/pluggable-discovery-state-machine.dot

@@ -0,0 +1,20 @@
+digraph {
+  // This generates pluggable-discovery-state-machine.png
+  // using the following command:
+  // dot -Tpng -Gsize=10,20\! pluggable-discovery-state-machine.dot > pluggable-discovery-state-machine.png
+  label="Pluggable discovery state machine"
+  labelloc=t
+  fontname="sans-serif"
+  node [fontname="sans-serif"]
+  edge [colorscheme=set15 fontsize=7 fontname="sans-serif"]
+  Alive -> Idling [label="  HELLO  " color=2 fontcolor=2]
+  Idling -> Running [label="  START  " color=3 fontcolor=3]
+  Running -> Idling [label="  STOP  " color=1 fontcolor=1]
+  Running -> Running [label="  LIST  " color=5 fontcolor=5]
+  Running -> Dead [label="  QUIT  " color=4 fontcolor=4]
+  Idling -> Syncing [label="  START_SYNC  " color=3 fontcolor=3]
+  Syncing -> Idling [label="  STOP  " color=1 fontcolor=1]
+  Syncing -> Dead [label="  QUIT  " color=4 fontcolor=4]
+  Idling -> Dead [label="  QUIT  " color=4 fontcolor=4]
+  Alive -> Dead [label="  QUIT  " color=4 fontcolor=4]
+}

docs/img/pluggable-discovery-state-machine.png

@@ -276,6 +276,18 @@ If the client sends an invalid or malformed command, the discovery should answer
 }
 ```
 
+### State machine
+
+A well behaved pluggable discovery tool must reflect the following state machine.
+
+![Pluggable discovery state machine](img/pluggable-discovery-state-machine.png)
+
+The arrows represent the commands outlined in the above sections, calling successfully a command assumes the state
+changes.
+
+A pluggable discovery state is Alive when the process has been started but no command has been executed. Dead means the
+process has been stopped and no further commands can be received.
+
 ### Board identification
 
 The `properties` associated to a port can be used to identify the board attached to that port. The algorithm is simple: