Add logo and link it to the RetroAchievement site

Author: cmitu

docs/RetroAchievements.md

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+![https://retroachievements.org](https://retroachievements.org/Images/RA_Logo10.png)
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 RetroAchievements are a way to log your achievements on oldschool console games. It is integrated into RetroArch and is only supported by select cores ([see list below](#supported-systems)).
 
 **IMPORTANT NOTE**: The RetroAchievements feature is continuously being worked and improved. Then if you downloaded (or even worse: bought) a custom RetroPie image, there's a chance you have an outdated RetroArch version and face compatibility issues with the achievements feature. For a better experience, it's recommended to use the [official RetroPie image](https://retropie.org.uk/download/) and keep RetroArch and libretro cores [up to date](https://retropie.org.uk/docs/Updating-RetroPie/#updatinginstalling-individual-packages).