You are a lucky man because this feature is already present in Openshot and since a while.
In fact as usual, there are not one but two ways to realize that in Openshot.
You have an easy way for the common users who is in the textual menu of the clip. And here, you chose your kind of rotation that you 'd like.
You can too apply an effect called rotation (in fact it is the same used) and open the Clip properties. At the last section/tab you select your effect (you can used several effects for one clip) and edit his properties. It is more for people who search something more precise same if you can do this operation easily i.e. you could choose for sample an rotation of 85° instead of a 90°)
You should take a look at our manual on the utilisation of Openshot that you have installed in the same time of Openshot.
You have too the HTML version of this one here : http://www.openshotusers.com/help/en/
You could check on this forum too the answers given at this question and see better how it is working.