RStudio installation: missing step in 'Na de eerste installatie'

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Step 7 under 'Na de eerste installatie' is not possible because knitr is not yet installed in a new installation (while following the instructions). This could simply be solved by adding a step to install knitr, but as this manual is mostly used by people that are not yet familiar with R, this step needs some more explanation than just 'installeer het package knitr'. Any suggestions how to describe this clearly for a starting R user?