docs: add simple usage example to README

Author: hellsontime

README.md

@@ -41,3 +41,36 @@ Download to a specific directory:
 ```bash
 scdl https://soundcloud.com/cowboyclicker/stay --output ~/Music/
 ```
+
+## Library Usage
+
+### Installation
+
+```bash
+go get github.com/hellsontime/scdl
+```
+
+### Simple Usage
+
+```go
+import "github.com/hellsontime/scdl"
+
+// ...
+
+client, err := scdl.NewClient()
+if err != nil {
+    return err
+}
+
+// Fetch track metadata
+track, err := client.GetTrack("https://soundcloud.com/cowboyclicker/stay")
+if err != nil {
+    return err
+}
+
+// Download the track to the current directory
+path, err := client.Download(track, ".", nil) // progress callback is optional
+if err != nil {
+    return err
+}
+```