Add sequence diagram

docs/UML/Sequence Diagrams/Cropping an Image.puml

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+@startuml
+'https://plantuml.com/sequence-diagram
+
+Actor User
+Boundary GUI
+
+User --> GUI : User selects crop
+GUI --> User : Dialogue box pops up asking for co-ordinates
+User --> GUI : User specifies co-ordinates
+GUI --> CropImage : manipulateImage()
+CropImage --> GUI : New image is created and sent back to the GUI
+GUI --> User : User sees new image
+
+@enduml