Added a note about OpenCV logo

doc/opencv-logo.md

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+OpenCV logo has been originally designed and contributed to OpenCV by Adi Shavit in 2006. The graphical part consists of three stylized letters O, C, V, colored in the primary R, G, B color components, used by humans and computers to perceive the world. It is shaped in a way to mimic the famous [Kanizsa's triangle](https://en.wikipedia.org/wiki/Illusory_contours) to emphasize that the prior knowledge and internal processing are at least as important as the actually acquired "raw" data.
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+The restyled version of the logo has been designed and contributed by [xperience.ai](https://xperience.ai/) in July 2020 for the [20th anniversary](https://opencv.org/anniversary/) of OpenCV.
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+The logo uses [Exo 2](https://fonts.google.com/specimen/Exo+2#about) font by Natanael Gama distributed under OFL license.
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+Higher-resolution version of the logo, as well as SVG version of it, can be obtained at OpenCV [Media Kit](https://opencv.org/resources/media-kit/).
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+![](./opencv-logo2.png)