Update main README file

Author: eloots

README.md

@@ -1,8 +1,11 @@
 # Scala 3 course CMT template
 
-This is a two exercise template for a scala 3 course.
+This is a three exercise template for a scala 3 course:
 
-The first exercise contains a Hello world application and
-doesn't use a build.
+- The first exercise is a "Hello, World" app
+- The second exercise is about calculating the sum
+  of prime numbers and requires the students to 'pull'
+  in code to generate prime numbers
+- The third exercise is about decomposing a given
+  integer number into its prime factors
 
-The second exercise add a minimal `sbt` build definition.