docs: update readme

add underline styling examples

Author: aymanbagabas

README.md

@@ -115,6 +115,13 @@ s.CrossOut()
 s.Underline()
 s.Overline()
 
+// Underline styles
+s.Underline(output.Color("#ff0000"))
+s.UnderCurl(output.Color("11"))
+s.UnderDouble()
+s.UnderDotted(output.Color("216"))
+s.UnderDashed()
+
 // Reverse swaps current fore- & background colors
 s.Reverse()