Clarify dependency on Matomo module

Problem/Motivation

It is not clear from if the Matomo contrib module is required or not.

According to the readme it is required, but the project page says it is optional.

The readme contains steps on configuring the Matomo module but not on installing it.

Steps to reproduce

Read the readme.

Proposed resolution

- Clarify the readme or make the module required in the info.yml / composer.json
- Explain in the project page and readme what the benefits are of optionally enabling the Matomo module

Remaining tasks

Do it

User interface changes

See above.

API changes

None

Data model changes

Nope