Add Link to Todo List (TodoMVC) TDD Example

[nelsonic]

Finally finished writing the Todo List TDD end-to-end example: https://github.com/dwyl/todomvc-vanilla-javascript-elm-architecture-example

Todo:

• [x] Add link to "what next" section: https://github.com/dwyl/learn-tdd#what-to-learn-next

[luqmanoop]

I'm on it

[nelsonic]

@codeshifu thanks for adding the link! 🥇