add notes about interpolation

README.md

@@ -26,6 +26,7 @@ Language structures used include:
 * Iteration
 * Actions (as in `file.forEachLine`)
 * Pattern matching (simple example using `when` expression)
+* String interpolation
 
 Kotlin library features used include:
 

src/three_feature.kt

@@ -45,6 +45,7 @@ fun main(args: Array<String>) {
         }
 
         // input and output from console are as easy as `println()` and `readline()`
+        // uses string interpolation for easier formatting
         println("$course:\n\t$letter ($average)\n")
     }
 }