Update README badges (#40)

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README.md

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 # WoodburyMatrices
 
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 This package provides support for the [Woodbury matrix identity](http://en.wikipedia.org/wiki/Woodbury_matrix_identity) for the Julia programming language.  This is a generalization of the [Sherman-Morrison formula](http://en.wikipedia.org/wiki/Sherman%E2%80%93Morrison_formula). Note that the Woodbury matrix identity is notorious for floating-point roundoff errors, so be prepared for a certain amount of inaccuracy in the result.