Create CITATION.bib (#98)

* Create CITATION.bib

Creates a minimal CITATION.bib

* add DOI and reformat

* remove tabs

[skip ci]

Should look better now, there seemed to have been some tabs introduced when copy-pasting.

CITATION.bib

@@ -0,0 +1,12 @@
+% If you wish to cite FFTW in a general publication, we recommend this paper:
+@Article{FFTW.jl-2005,
+  author   =  {Frigo, Matteo and Johnson, Steven~G.},
+  title    =  {The Design and Implementation of {FFTW3}},
+  journal  =  {Proceedings of the IEEE},
+  year     =  2005,
+  volume   =  93,
+  number   =  2,
+  pages    =  {216--231},
+  note     =  {Special issue on ``Program Generation, Optimization, and Platform Adaptation''},
+  doi      =  {10.1109/JPROC.2004.840301}
+}