docs: add link to contribution guide in README (#1785)

Author: braaar

README.md

@@ -40,6 +40,7 @@ Class-validator works on both browser and node.js platforms.
   - [Samples](#samples)
   - [Extensions](#extensions)
   - [Release notes](#release-notes)
+  - [Contributing](#contributing)
 
 ## Installation
 
@@ -933,3 +934,7 @@ See information about breaking changes and release notes [here][3].
 [1]: https://github.com/chriso/validator.js
 [2]: https://github.com/pleerock/typedi
 [3]: CHANGELOG.md
+
+## Contributing
+
+For information about how to contribute to this project, see [TypeStack's general contribution guide](https://github.com/typestack/.github/blob/master/CONTRIBUTING.md).