improve comments and readme

README.md

@@ -10,3 +10,20 @@ Create a program (or programs) which show how the language you picked (Kotlin) w
 
 ## Report Card Generator
 
+Accepts a file `sample.grades` where each line is an assignment in the format: `<course name>: <grade>`. The program iterates through the file and computes the average and letter grades for all courses listed in `sample.grades`.
+
+Language structures used include:
+
+* Iterable unpacking
+* Implicit type arguments
+* Null safety and propagation (as in `courses[course]!!.add(grade)`)
+* Iteration
+* Actions (as in `file.forEachLine`)
+* Pattern matching (simple example using `when` expression)
+
+Kotlin library features used include:
+
+* File I/O
+* Standard I/O
+* String conversions
+* Mutable maps and lists