add usage example

paper/paper.md

@@ -23,6 +23,7 @@ DNA shotgun sequencing of human, animal, and environmental samples has opened up
 One aspect of metagenomics is investigating the community composition of organisms within a sequencing sample with tools known as taxonomic classifiers, such as Kraken @kraken.
 
 In cases where the origin of a metagenomic sample, its source, is unknown, it is often part of the research question to predict and/or confirm the source.
+For example, in microbial archaelogy, it is sometimes necessary to rely on metagenomics to validate the source of paleofaeces.
 Using samples of known sources, a reference dataset can be established with the taxonomic composition of the samples, *i.e.* the organisms identified in the samples as features, and the sources of the samples as class labels.
 With this reference dataset, a machine learning algorithm can be trained to predict the source of unknown samples (sinks) from their taxonomic composition.  
 Other tools used to perform the prediction of a sample source already exist, such as SourceTracker [@sourcetracker], which employs Gibbs sampling.