add reference

Author: simonbyrne

README.md

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 Implements Singular Value Decomposition for generic number types, such as `BigFloat` and `Complex{BigFloat}`. It internally overloads several Base functions such that existing methods (`svd`, `svdfact` and `svdvals`) should work directly.
 
 It uses a Golub-Kahan 2-stage algorithm of bidiagonalization with Householder reflections, followed by an implicit QR with shift.
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+## References
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+* Golub, G. H. and Van Loan, C. F. (1996), "§8.6.2 The SVD Algorithm", *Matrix Computations* (3rd Ed.), Johns Hopkins University Press, Baltimore, MD, USA.