Add mermaid diagram to README

[felixfbecker]

I took a stab at modelling the current and desired state so we can include it in the README:

graph LR
  State --> CodeView[Code view]
  CodeView --renders with state--> HoverOverlay
  CodeView --passes events--> locateTarget
  subgraph Hoverifier
  locateTarget --> fetching[Hover and Definition Fetching]
  fetching --> State
  end

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graph LR
  State --> CodeView[Code view]
  CodeView --renders with state--> HoverOverlay
  HoverOverlay --> Highlight[Highlight box]
  CodeView --passes events--> PositionLogic[Position logic]
  PositionLogic --returns positions--> CodeView
  CodeView --passes positions--> fetching[Hover and Definition Fetching]
  subgraph State Management
  fetching --> State
  end

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@ijsnow are these accurate?