Clarify that both crate and super are keywords (#236)

Author: LukeMathWalker

book/src/03_ticket_v1/03_modules.md

@@ -85,6 +85,9 @@ You can compose the path in various ways:
 - starting from the parent module, e.g. `super::my_function`
 - starting from the current module, e.g. `sub_module_1::MyStruct`
 
+Both `crate` and `super` are **keywords**.\
+`crate` refers to the root of the current crate, while `super` refers to the parent of the current module.
+
 Having to write the full path every time you want to refer to a type can be cumbersome.
 To make your life easier, you can introduce a `use` statement to bring the entity into scope.