updating citation to BMC RACS paper

Please cite the following paper when ever you are using RACS:
- "RACS: Rapid Analysis of ChIP-Seq data for contig based genomes",
  Saettone et al., BMC Bioinformatics 20, 533 (2019) doi:10.1186/s12859-019-3100-2
  -- https://arxiv.org/abs/1905.02771