Add cargo expand command example (#31)

* Add cargo expand command example

It took me some time to figure out how `cargo expand` works.

* Use code block for comment

Author: pickfire

jira-wip/src/koans/01_ticket/07_derive.rs

@@ -14,6 +14,10 @@ mod derive {
     /// You can check the code generated by derive macros using `cargo expand`:
     /// https://github.com/dtolnay/cargo-expand
     ///
+    /// ```sh
+    /// cargo expand -p jira-wip path_to_enlightenment::derive
+    /// ```
+    ///
     /// PartialEq is not the only trait whose implementation can be derived automatically!
     #[derive(PartialEq)]
     pub enum Status {