Add usage instructions to README.

[mrcjkb]

Please check if the PR fulfills these requirements

• [x] The PR has no duplicates (please search among the Pull Requests before creating one)

• [x] The PR follows our contributing guidelines

• [Tests for the changes have been added (for bug fixes / features) --> No code changes

• What kind of change does this PR introduce? Docs update (README)

• What is the current behavior? No usage instructions in README

• What is the new behavior? Usage instructions in README

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[CLAassistant]

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