Developers use and contribute to repositories on GitHub. Documentation present in the repositories serves as an important source by helping developers to understand, maintain and contribute to the project. Currently, documentation in a repository is diversified, among various files, with most of it present in ReadMe files. However, other software artifacts in the repository, such as issue reports and pull requests could also contribute to documentation, without documentation being explicitly specified. Hence, in this paper, we propose a taxonomy of documentation sources by analyzing different software artifacts, developer interviews and card-sorting approach. We inspected multiple artifacts of 950 public GitHub repositories, written in four different programming languages, C++, C#, Python and Java, and analyzed the type and amount of documentation that could be extracted from these artifacts. To this end, we observe that, about 25.93% of information extracted from all sources proposed in the taxonomy contains error-related documentation, and that pull requests contribute to around 18.21% of extracted information.