Merge pull request #1610 from abelgvidal/fixcontrib

Amend link to Contribution Guide in README

README.md

@@ -31,7 +31,7 @@ To get up and running quickly, or as an alternative to running your own `step-ca
 [Documentation](https://smallstep.com/docs) |
 [Installation](https://smallstep.com/docs/step-ca/installation) |
 [Getting Started](https://smallstep.com/docs/step-ca/getting-started) |
-[Contributor's Guide](./docs/CONTRIBUTING.md)
+[Contributor's Guide](./CONTRIBUTING.md)
 
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