Added the citation clause.
Citing CCL
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-We have a white paper on the way.
+The CCL is still under development and should be considered research in progress. You are welcome to re-use the code, which is open source and available under terms consistent with `BSD 3-Clause <https://opensource.org/licenses/BSD-3-Clause>`_ licensing. If you make use of any of the ideas or software in this package in your own research, please cite them as "(LSST DESC, in preparation)" and provide a link to this repository: https://github.com/LSSTDESC/CCL. For free use of the `CLASS` library, the `CLASS` developers require that the `CLASS` paper be cited: CLASS II: Approximation schemes, D. Blas, J. Lesgourgues, T. Tram, arXiv:1104.2933, JCAP 1107 (2011) 034. The `CLASS` repository can be found in http://class-code.net. If you have comments, questions, or feedback, please `write us an issue <https://github.com/LSSTDESC/CCL/issues>`_. Finally, CCL uses code from the `FFTLog <http://casa.colorado.edu/~ajsh/FFTLog/>`_ package. We have obtained permission from the FFTLog author to include modified versions of his source code.
-IN DEVELOPMENT
