Update README.md

Author: pgleeson

README.md

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 # Presentations
-Presentations related to COMBINE standards
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+Presentations related to the [COMBINE standards](http://co.mbine.org/standards) and the [COMBINE network](http://co.mbine.org/about) in general.
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+For a quick overview can be found in the [COMBINE 2020 introductory slides](https://github.com/combine-org/Presentations/blob/master/IntroductionToCOMBINE2020.pdf).