docs: Add use citation from simplified likelihoods ATLAS PUB note (#1614

)

* Add use citation from 'Implementation of simplified likelihoods in HistFactory for searches for supersymmetry'
   - ATLAS PUB note: ATL-PHYS-PUB-2021-038
   - c.f. https://cdsweb.cern.ch/record/2782654

docs/bib/use_citations.bib

@@ -1,3 +1,17 @@
+% 2021-09-30
+@booklet{ATL-PHYS-PUB-2021-038,
+      author        = "{ATLAS Collaboration}",
+      title         = "{Implementation of simplified likelihoods in HistFactory
+                       for searches for supersymmetry}",
+      institution   = "CERN",
+      address       = "Geneva",
+      number        = "ATL-PHYS-PUB-2021-038",
+      month         = "Sep",
+      year          = "2021",
+      reportNumber  = "ATL-PHYS-PUB-2021-038",
+      url           = "https://cds.cern.ch/record/2782654"
+}
+
 % 2021-09-17
 @article{Baker:2021llj,
     author = "Baker, Michael J. and Faroughy, Darius A. and Trifinopoulos, Sokratis",