docs(readme): link to contributing (#419)

Author: AndersDJohnson

README.md

@@ -1,5 +1,5 @@
 ### Commitizen for contributors
-When you commit with Commitizen, you'll be prompted to fill out any required commit fields at commit time. No more waiting until later for a git commit hook to run and reject your commit (though [that](https://github.com/kentcdodds/validate-commit-msg) can still be helpful). No more digging through CONTRIBUTING.md to find what the preferred format is. Get instant feedback on your commit message formatting and be prompted for required fields.
+When you commit with Commitizen, you'll be prompted to fill out any required commit fields at commit time. No more waiting until later for a git commit hook to run and reject your commit (though [that](https://github.com/kentcdodds/validate-commit-msg) can still be helpful). No more digging through [CONTRIBUTING.md](CONTRIBUTING.md) to find what the preferred format is. Get instant feedback on your commit message formatting and be prompted for required fields.
 
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