docs(README): add custom writer usage example and clarify output options

Author: yarlson

README.md

@@ -38,6 +38,22 @@ defer cancel()
 p.Stop("Done!")
 ```
 
+## Custom Output Writer
+
+You can direct spinner output to an alternative writer (for example, `os.Stderr`) using the `WithWriter` option:
+
+```go
+p := pin.New("Processing...",
+    pin.WithSpinnerColor(pin.ColorCyan),
+    pin.WithTextColor(pin.ColorYellow),
+    pin.WithWriter(os.Stderr), // output will be written to stderr
+)
+cancel := p.Start(context.Background())
+defer cancel()
+// perform your work
+p.Stop("Done!")
+```
+
 ## Non-interactive Behavior
 
 When the spinner detects that `stdout` is not connected to an interactive terminal (for example, when output is piped), it disables animations and outputs messages as plain text. In this mode:

pin.go

@@ -45,6 +45,17 @@
 //	defer cancel()
 //	// ... do some work ...
 //	p.Fail("Error occurred")
+//
+// Example with custom output writer:
+//
+//	p := pin.New("Saving Data",
+//	    WithSpinnerColor(ColorMagenta),
+//	    WithWriter(os.Stderr), // send output to stderr
+//	)
+//	cancel := p.Start(context.Background())
+//	defer cancel()
+//	// ... do some work ...
+//	p.Stop("Saved!")
 package pin
 
 import (