docs: add TODO from section 7

Author: alexsavio

README.md

@@ -1,3 +1,14 @@
 # From zero to production in Rust
 
 Check the Justfile for the commands to run the project.
+
+## TODO:
+
+### Section 7
+
+- [ ] What happens if a user tries to subscribe twice? Make sure that they receive two confirmation emails;
+- [ ] What happens if a user clicks on a confirmation link twice?
+- [ ] What happens if the subscription token is well-formatted but non-existent?
+- [ ] Add validation on the incoming token, we are currently passing the raw user input straight into a query (thanks sqlx for protecting us from SQL injections <3);
+- [ ] Use a proper templating solution for our emails (e.g. tera);
+- [ ] Anything that comes to your mind!