Create CITATION.md (#4890)

As per #4213 (comment)

CITATION.md

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+# Citing `Cromwell`
+
+If you use `Cromwell` in your work we would prefer it if you would use the following reference in your work.
+
+## BibTeX
+
+```bibtex
+@misc{https://doi.org/10.7490/f1000research.1114634.1,
+  doi = {10.7490/f1000research.1114634.1},
+  url = {https://f1000research.com/slides/6-1381},
+  author = {Voss,  Kate and Auwera,  Geraldine Van Der and Gentry,  Jeff},
+  title = {Full-stack genomics pipelining with GATK4 + WDL + Cromwell [version 1; not peer reviewed]},
+  journal = {ISCB Comm J},
+  publisher = {F1000Research},
+  type = {slides}
+  volume = {6},
+  number = {1381},
+  year = {2017}
+}
+```
+
+## Textual
+
+Voss K, Van der Auwera G and Gentry J. Full-stack genomics pipelining with GATK4 + WDL + Cromwell
+[version 1; not peer reviewed]. _F1000Research_ 2017, **6**(ISCB Comm J):1381 (slides)
+(https://doi.org/10.7490/f1000research.1114634.1)