Update CITATION.cff to add 'preferred-citation'.

CITATION.cff

@@ -33,3 +33,27 @@ abstract: >-
 keywords:
   - bioinformatics
 license: BSD-3-Clause
+preferred-citation:
+  type: article
+  authors:
+    - given-names: Peter
+      family-names: Briggs
+    - given-names: A. Louise
+      family-names: Hunter
+      orcid: 'https://orcid.org/0000-0002-3874-4852'
+    - given-names: Shen-hsi
+      family-names: Yang
+    - given-names: Andrew D.
+      family-names: Sharrocks
+    - given-names: Mudassar
+      family-names: Iqbal
+      orcid: 'https://orcid.org/0000-0002-5006-4331'
+  doi: "https://doi.org/10.12688/f1000research.53926.2"
+  journal: "F1000Research"
+  title:  >-
+    PEGS: An efficient tool for gene set enrichment within
+    defined sets of genomic intervals [version 2; peer review:
+    2 approved]
+  issue: 579
+  volume: 10
+  year: 2021