Updated JOSS banner status

Author: sambitdash

README.md

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 [![codecov.io](http://codecov.io/github/sambitdash/PDFIO.jl/coverage.svg?branch=master)](http://codecov.io/github/sambitdash/PDFIO.jl?branch=master)
 [![Doc Latest](https://img.shields.io/badge/docs-latest-blue.svg)](https://sambitdash.github.io/PDFIO.jl/dev)
 [![Doc Stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://sambitdash.github.io/PDFIO.jl/stable)
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 PDFIO is a native Julia implementation for reading PDF files. It's an 100% Julia 
 implementation of the PDF specification. Other than a few well established