Interesting insights have been found by the research community indicating that early user involvement in Requirements Engineering (RE) has a considerable association with higher requirements quality, software project success and as well boosting user loyalty. In addition, traditional RE approaches confront scalability issues and would be time consuming and expensive to be applied with contemporary applications that can be surrounded by a large crowd. Therefore, recent attention has been shed on leveraging the principle of Crowdsourcing (CS) in requirements engineering. Engaging the crowd in RE activities has been researched by several studies. Hence, we synthesize and review the literature of the knowledge domain Crowdsourcing Requirements Engineering using a proposed taxonomy of the area. A total of 52 studies were selected for review in this paper. The review aims to provide the potential directions in the area and pave the way for other researchers to understand it and find possible gaps.