Loopback-next: Update docs: Remind users they may need to add tests

[cloudwheels]

Description / Steps to reproduce / Feature proposal

• Work through the steps to build todoList on top of todos

Current Behavior

• If the todoList tutorial is pulled from the repo / using lb4 example it will include tests.
• Tests are not automatically created if starting with the todo example and building todoList using lb4 controller etc

Expected Behavior

• Update docs: Remind users they may need to add tests
• Make it easy to add existing tests (or use this opportunity for a tutorial-within-tutorial on building tests) - see #885

[bajtos]

I agree we should improve lb4 tooling to help users to write tests for their code too. I think the following issue is pretty much covering your second point: _#885 "lb4 controller" should prepare test infrastructure_

Let's keep this issue focused on short-term improvements of our docs, i.e. _Update docs: Remind users they may need to add tests_.

@cloudwheels would you like to contribute this change yourself?

[cloudwheels]

Updated title to reflect your suggestions as basis for straightforward PR - yes can take this :)
Will get myself organised to contrib asap, may need a day or two if that's OK to balance things with the real world (mucho tiempo aqui!) but no longer...