Add 'Start train session diagramm'

Start-train-session-diagramm.md

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+```plantuml
+@startuml
+start
+fork
+  :find periferals;
+fork again
+  :get previous connected sensors state;
+end merge
+if(have some previously connected sensors) then (yes)
+  :connect saved;
+else (no)
+  :connect closest;
+endif
+if(all sensors connected) then (yes)
+  label connect
+  :can load workout;
+else (no)
+  :add manually;
+  goto connect
+endif
+stop
+@enduml
+```