Update README.md
Add Streets of Rage 2X preview video.
OpenBOR is a continuation of the Beats Of Rage 2D game engine, which was originally
created by the wonderful folks over at [Senile Team](http://www.senileteam.com).
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+[![Streets of Rage2X](http://img.youtube.com/vi/TbZD1pWsJeI/0.jpg)](https://www.youtube.com/watch?v=TbZD1pWsJeI "Streets of Rage2X Preview")
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## History
In 2004, [Senile Team](http://www.senileteam.com) released Beats of Rage, a free beat-'em-up for DOS inspired
by SEGA's Streets of Rage series and using sprites from SNK Playmore's King of
