Add citation as CFF file.

[skip ci]

Author: maleadt

Diff for: CITATION.bib

@@ -0,0 +1,16 @@
+cff-version: 1.2.0
+message: If you use CUDA.jl in your work, please cite it using the following metadata
+title: CUDA.jl
+abstract: This package provides CUDA programming support for the Julia programming language
+authors:
+- family-names: Besard
+  given-names: Tim
+  affiliation: Julia Computing
+keywords:
+- julia
+- cuda
+doi: 10.5281/zenodo.1184077
+date-released: 2018-07-29
+license: Apache-2.0
+repository-artifact: https://rubygems.org/gems/cff
+repository-code: https://github.com/citation-file-format/ruby-cff