added example for library usage

Author: keith-turner

README.md

@@ -113,6 +113,8 @@ go build ecoji.go
 ./ecoji --help
 ```
 
+For an example of how to use Ecoji as library see [library-example.md](docs/library-example.md).
+
 [emoji]: https://unicode.org/emoji/
 [video]: https://www.youtube.com/watch?v=XCsL89YtqCs
 [tour]: https://tour.golang.org/welcome/1

docs/library-example.md

@@ -0,0 +1,81 @@
+# Using Ecoji as a library
+
+This is an example that shows how to use Ecoji as a library.  Start off with a new directory.
+
+```bash
+mkdir /tmp/ecoji-example
+cd /tmp/ecoji-example
+```
+
+Then create a file called `go.mod` in that directory with the following contents.
+
+```
+go 1.18
+
+module local/ecoji-example
+
+require github.com/keith-turner/ecoji/v2 v2.0.0
+
+```
+
+Then create file named `ecoji-example.go` with the following contents.  This example program will read a file, encode with Ecoji V2, and write that to a new file.
+
+```go
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"github.com/keith-turner/ecoji/v2"
+	"log"
+	"os"
+)
+
+func main() {
+
+	if len(os.Args) != 3 {
+		fmt.Printf("Usage : %s <input file> <output file>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	input := os.Args[1]
+	output := os.Args[2]
+
+	infile, err1 := os.OpenFile(input, os.O_RDONLY, 0)
+	if err1 != nil {
+		log.Fatal(err1)
+	}
+
+	outfile, err2 := os.Create(output)
+	if err2 != nil {
+		log.Fatal(err2)
+	}
+
+	outbuf := bufio.NewWriter(outfile)
+
+	//encode data using Ecoji V2 with a 72 emoji wrap
+	if err := ecoji.EncodeV2(bufio.NewReader(infile), outbuf, 72); err != nil {
+		log.Fatal(err)
+	}
+
+	infile.Close()
+	outbuf.Flush()
+	outfile.Close()
+}
+```
+
+Then run the following commands to build the executable.
+
+```
+go mod download github.com/keith-turner/ecoji/v2
+go build ecoji-example.go
+```
+
+Now you should be able to use the executable to encode a file.  Below is an example if running it.
+
+```
+/tmp/ecoji-example$ ./ecoji-example go.mod test.ecoji
+/tmp/ecoji-example$ cat test.ecoji 
+👮😽♏🧧🎌🤭🪜🕋💎🧵🐬🌭🍉😑🦎🛸💁🙁🐩🤜👺🕺🛫👉👖😢⛲🌭🛞🍯🍡👂💦🪳🍡🏮👮🪳🏨🌽👙😱🎨🤚🎅😁🐫👅👱😢🏫👃💊🔪🍓🤒👞🙁🖖🐀💩🚞⛵🔅🎀🙎🤹♍🛞☕
+
+```