Todo explanation with example

book/src/language-tour/todo.md

@@ -1 +1,12 @@
 # Todo
+
+A 'todo' type errors on evaluation but casts to any type.
+It can be useful to let your project typecheck while you are still working on parts of it.
+
+```
+fn notImplementedYet(){
+    todo
+}
+```
+
+It is also good practice to use instead of a todo comment.