create CITATION.cff (#1029)

#1021 please look at the file and let me know if I should make any changes

Co-authored-by: Benjamin Pedigo <benjamindpedigo@gmail.com>

CITATION.cff

@@ -0,0 +1,39 @@
+cff-version: 1.2.0
+message: "If you find graspologic useful in your work, please cite the package via the GraSPy paper"  
+authors:
+  - family-names: "Chung"
+    given-names: "Jaewon"
+  - family-names: "Pedigo"
+    given-names: "Benjamin D."
+  - family-names: "Bridgeford"
+    given-names: "Eric W."
+  - family-names: "Varjavand"
+    given-names: "Bijan K."
+  - family-names: "Helm"
+    given-names: "Hayden S."
+  - family-names: "Vogelstein"
+    given-names: "Joshua T."
+title: "GraSPy: Graph Statistics in Python"
+version: 3.0.0
+url: "https://github.com/microsoft/graspologic"
+preferred-citation:
+  type: software
+  issue: 158
+  volume: 20
+  journal: "Journal of Machine Learning Research"
+  authors:
+  - family-names: "Chung"
+    given-names: "Jaewon"
+  - family-names: "Pedigo"
+    given-names: "Benjamin D."
+  - family-names: "Bridgeford"
+    given-names: "Eric W."
+  - family-names: "Varjavand"
+    given-names: "Bijan K."
+  - family-names: "Helm"
+    given-names: "Hayden S."
+  - family-names: "Vogelstein"
+    given-names: "Joshua T."
+  start: 1
+  end: 7
+  year: 2019