Fix wrong link in readme

[ClemensLinnhoff]

Reference to a related issue in the repository

#63

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Removed dead link in readme and added explanation about generating rules.

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@raue

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• My code and comments follow the contributors guidelines of this project.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation for osi-validation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests / travis ci pass locally with my changes.