dev guide: mention that example component enhancements are lacking tests

[damithc]

The example code we give for component enhancements don't have tests. We should mention that in the dev guide and ask the reader to implement tests as well.
Alternatively, we can add the missing tests to those PRs.

[Rinder5]

Currently the rest of the component enhancements seem to be updated. Only one left is this:
https://github.com/se-edu/addressbook-level4/blob/master/docs/DeveloperGuide.adoc#storage-component