Add readme for all plugins

[Ni55aN]

https://github.com/retejs/rete/issues/80#issuecomment-513693187

Also attach screenshots if possible (where to store screen screenhots? In the repository itself it is a bad idea)

[Ni55aN]

List of repos without an info in the readme

• https://github.com/retejs/area-plugin
• https://github.com/retejs/task-plugin
• https://github.com/retejs/connection-reroute-plugin
• https://github.com/retejs/profiler-plugin
• https://github.com/retejs/minimap-plugin
• https://github.com/retejs/keyboard-plugin

[rete-js[bot]]

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