docs: add dependencies installation info, remove test-requirements.txt

[dorkamotorka]

SUMMARY

Small update to the root README.md to add information to install the dependencies in the requirements.txt.

Also I'd suggest to either remove test-requirements.txt, because all dependencies are already specified in the requirements.txt or we could also remove the "test" requirements from the requirements.txt and add it to the test-requirements.txt?

ISSUE TYPE

• Docs Pull Request

COMPONENT NAME

README.md

[softwarefactory-project-zuul]

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