Define sub-modules interactions

[Sinclert]

This PR updates the Makefile, README and reana/workflow.yml with the necessary changes after the introduction of GIT sub-modules to manage sub-workflow repositories: madminer-workflow-ph and madminer-workflow-ml.

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~In addition, it addresses issue #30 by introducing template render Makefile rule for the ML sub-workflow, allowing it to both:~

~Update:~

~In addition, it addresses issue #30 by moving the last step of the Physics sub-workflow (combine) into this repository workflow.yml, so that it can be referenced from the ML sub-workflow. This change allows both:~

Update 2:

In addition, it addresses issue #30 by concatenating the Physics and ML sub-workflows directly on this repo workflow.yml. This change allows both:

• Individually test both madminer-workflow-ml and madminer-workflow-ph Yadage sub-workflow.
• Jointly test the Yadage workflow on this repository.

[irinaespejo]

Thank you! Is it now ready for merge?

[Sinclert]

Yes, it is 🚀