add citation

Author: nggvs

CITATIONS.md

@@ -18,6 +18,8 @@
 
 > Ewels P, Magnusson M, Lundin S, Käller M. MultiQC: summarize analysis results for multiple tools and samples in a single report. Bioinformatics. 2016 Oct 1;32(19):3047-8. doi: 10.1093/bioinformatics/btw354. Epub 2016 Jun 16. PubMed PMID: 27312411; PubMed Central PMCID: PMC5039924.
 
+- [Seqtk](https://github.com/lh3/seqtk)
+
 ## Software packaging/containerisation tools
 
 - [Anaconda](https://anaconda.com)