chore(README): add `npm install` hint

[barmac]

This change is a temporary fix for https://github.com/bpmn-io/dmn-testing-plugin/issues/15

Related to https://github.com/bpmn-io/dmn-testing-plugin/issues/17

Definition of Done

• [ ] corresponds to the design principles
• [ ] corresponds to the code standards
• [ ] passes Continuous Integration checks
• [ ] available as feature branch on GitHub
  • [ ] contains the cleaned up commit history
  • [ ] commit messages satisfy our commit message guidelines
  • [ ] a single commit closes the issue via Closes #issuenr

[MaxTru]

Good stuff! We should definetly add this hint