Add readme.

Author: icedream

README.md

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+# bsdiff for Go
+
+This wrapper implementation for Golang reuses the existing
+C version of bsdiff as provided by @mendsley and wraps it
+into a Go package, abstracting away all the cgo work that
+would need to be done otherwise.
+
+## Installation
+
+The library and the helper binaries `go-bsdiff` and `go-bspatch` can be installed like this:
+
+    go get -v github.com/icedream/go-bsdiff/...
+
+## Usage in application code
+
+For exact documentation of the library check out [GoDoc](https://godoc.org/github.com/icedream/go-bsdiff).
+
+Library functionality is provided both as a package `bsdiff` containing both
+methods `Diff` and `Patch`, or as subpackages `diff` and `patch` which each
+only link the wanted functionality.
+
+Below example will generate a patch and apply it again in one go. This code
+is not safe against errors but it shows how to use the provided routines:
+
+```go
+package main
+
+import (
+    "os"
+    "github.com/icedream/go-bsdiff"
+    // Or use the subpackages to only link what you need:
+    //"github.com/icedream/go-bsdiff/diff"
+    //"github.com/icedream/go-bsdiff/patch"
+)
+
+const (
+    oldFilePath = "your_old_file.dat"
+    newFilePath = "your_new_file.dat"
+    patchFilePath = "the_generated.patch"
+)
+
+func generatePatch() error {
+    oldFile, _ := os.Open(oldFilePath)
+    defer oldFile.Close()
+    newFile, _ := os.Open(newFilePath)
+    defer newFile.Close()
+    patchFile, _ := os.Create(patchFilePath)
+    defer patchFile.Close()
+
+    return bsdiff.Diff(oldFile, newFile, patchFile)
+}
+
+func applyPatch() error {
+    oldFile, _ := os.Open(oldFilePath)
+    defer oldFile.Close()
+    newFile, _ := os.Create(newFilePath)
+    defer newFile.Close()
+    patchFile, _ := os.Open(patchFilePath)
+    defer patchFile.Close()
+
+    return bsdiff.Patch(oldFile, newFile, patchFile)
+}
+
+func main() {
+    generatePatch()
+    applyPatch()
+}
+```
+
+## Usage of the tools
+
+The tools `go-bsdiff` and `go-bspatch` both provide a `--help` flag to print
+out all information but in their simplest form, they can be used like this:
+
+```sh
+# Creates a patch file $the_generated with differences from
+# $your_old_file to $your_new_file.
+go-bsdiff "$your_old_file" "$your_new_file" "$the_generated"
+
+# Applies a patch file $the_generated on "your_old_file
+# and saves the new file to $your_new_file.
+go-bspatch "$your_old_file" "$your_new_file" "$the_generated"
+```
+
+## Motivation
+
+There is [a Go implementation of an older version of bsdiff called binarydist](https://github.com/kr/binarydist). The original bsdiff tool has since been updated so patches generating using the original tool are no longer compatible with the Go implementation. I don't know what the changes between the versions are and unfortunately I don't have the time to search for these changes and port them over as a pull request, otherwise I'd have done that instead.
+
+Additionally, @mendsley has already done the extra work of rewriting the code to be embeddable in any application code as a library. So why not make use of cgo, which I was going to look into in more detail at some point anyways?