Add credits for icon sources in README

Added credits section for icon sources.

Author: kodpe

README.md

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 # SimpleOverlay
 Lightweight native C++ Win32 simple overlay tool with multi-monitor support, live controls, and image rendering.
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+#### Credits (Icons)
+<a href="https://www.flaticon.com/fr/auteurs/see-icons">See Icons - Licence Flaticon</a>
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+<a href="https://www.flaticon.com/fr/auteurs/freepik">Freepik - Licence Flaticon</a>
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+<a href="https://www.flaticon.com/authors/laisa-islam-ani">Laisa Islam Ani - Licence Flaticon</a>
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+<a href="https://www.flaticon.com/authors/hilmy-abiyyu-a">Hilmy Abiyyu A. - Licence Flaticon</a>
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+