Out of all the ways music can be written down, rhythm is probably the least detailed. Most of the time it is just written as chords with lines separating each bar as shown below.
Stand By Me
As you can see this method doesn't show what rhythm pattern to use, how many beats per bar or at what tempo to play the song. So why is it written like this? One of the reasons is because rhythm fits into set styles and patterns, and there are really only a certain number of main styles you can play. Sure there's lots of variation in each of those styles but the differences are usually minimal. For example, so far you have learnt two different rhythm patterns. Those two patterns fit into one main style, "Eighth Note Rhythms". This is where you're strumming twice every beat, down on the beat and up on the off beat.
Most of the time chord charts are written out based on assumed knowledge, that is it is assumed that you either know the song or know how to play rhythm already.