added codecov badge

README.md

@@ -4,6 +4,7 @@ Pure Go implementation of [bsdiff](http://www.daemonology.net/bsdiff/) 4.
 [![GoDoc](https://godoc.org/github.com/gabstv/go-bsdiff?status.svg)](https://godoc.org/github.com/gabstv/go-bsdiff)
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 bsdiff and bspatch are tools for building and applying patches to binary files. By using suffix sorting (specifically, Larsson and Sadakane's [qsufsort](http://www.larsson.dogma.net/ssrev-tr.pdf)) and taking advantage of how executable files change.