Update README.md

Author: patrickfav

README.md

@@ -11,6 +11,7 @@ HKDF specifies a version of
 the [NIST Special Publication 800-56C](https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Cr1.pdf) "
 Two-Step Key Derivation" scheme.
 
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 [![Download](https://api.bintray.com/packages/patrickfav/maven/hkdf/images/download.svg)](https://bintray.com/patrickfav/maven/hkdf/_latestVersion)
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