Add example in README.md

Signed-off-by: Ulysses Souza <[email protected]>

README.md

@@ -13,6 +13,68 @@ It's meant to be configurable through a simple API that lets you define your own
 go get -u github.com/ulyssessouza/envlang
 ```
 
+### Usage
+
+Given the following environment file `input.env`:
+```env
+FOO=foo
+BAR="bar"
+BAZ = 1
+EMPTY=
+UNSET
+
+VAR1=$FOO
+VAR2=${BAR}
+VAR3=${EMPTY:-default_value_for_empty}
+VAR4=${UNSET-default_value_for_unset}
+```
+
+Example code:
+```go
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/ulyssessouza/envlang"
+	"github.com/ulyssessouza/envlang/dao"
+)
+
+func main() {
+	file, err := os.Open("input.env")
+	if err != nil {
+		return
+	}
+	d := dao.NewDefaultDaoFromMap(nil)
+	vars := envlang.GetVariablesFromInputStream(d, file)
+	fmt.Printf("FOO=%q\n", *vars["FOO"])
+	fmt.Printf("BAR=%q\n", *vars["BAR"])
+	fmt.Printf("BAZ=%q\n", *vars["BAZ"])
+	fmt.Printf("EMPTY=%q\n", *vars["EMPTY"]) // Use %q to print empty strings
+	fmt.Printf("UNSET=%q\n", vars["UNSET"])
+	fmt.Printf("VAR1=%q\n", *vars["VAR1"])
+	fmt.Printf("VAR2=%q\n", *vars["VAR2"])
+	fmt.Printf("VAR3=%q\n", *vars["VAR3"])
+	fmt.Printf("VAR4=%q\n", *vars["VAR4"])
+}
+```
+
+Output:
+```
+FOO="foo"
+BAR="bar"
+BAZ="1"
+EMPTY=""
+UNSET=%!q(*string=<nil>)
+VAR1="foo"
+VAR2="bar"
+VAR3="default_value_for_empty"
+VAR4="default_value_for_unset"
+```
+
+Please note that it used `%q` to print empty strings, as the default `fmt.Printf` will not print them.
+
 ## License
 
 This project is licensed under the [MIT](LICENSE) License - see the LICENSE.md file for details