add stable badge

Author: Will Kimmerer

README.md

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 [![](https://img.shields.io/badge/docs-dev-blue.svg)](https://graphblas.juliasparse.org/dev/)
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+[![](https://img.shields.io/badge/docs-stable-green.svg)](https://graphblas.juliasparse.org/stable/)
 # SuiteSparseGraphBLAS.jl
 A fast, general sparse linear algebra and graph computation package, based on SuiteSparse:GraphBLAS.