Add more explanation

lesson-templates/lesson-01-hello-world.tm

@@ -1,7 +1,11 @@
 # This is a simple "Hello world" program.
 
+# Lines beginning with a '#' are comments
+
 # Programs need a main() function to run:
 func main():
 
+    # The language is indentation-sensitive, so indentation matters!
+
     # Edit this code so it prints the text "Hello world"
     say("Goodbye moon")