Document engine-specific excludes

While we prefer to keep documentation on docs.codeclimate.com, this
engine is the most frequent beneficiary of this general configuration
point. Therefore, it's useful to improve visibility by mentioning it in the
project README as well.

Author: Mange

README.md

@@ -68,7 +68,22 @@ engines:
 Note that you have the update the YAML structure under the `languages` key to
 the Hash type to support extra configuration.
 
+### Excluding files and directories
+
+As with any other Code Climate engine, you can exclude certain files or
+directories from being analyzed. For more information, see
+[*Exclude paths for specific engines*][exclude-files-engine] in our
+documentation.
+
+```yaml
+engines:
+  duplication:
+    exclude_paths:
+    - examples/
+```
+
 [codeclimate]: https://codeclimate.com/dashboard
 [what-is-duplication]: https://docs.codeclimate.com/docs/duplication-concept
 [flay]: https://github.com/seattlerb/flay
 [cli]: https://github.com/codeclimate/codeclimate
+[exclude-files-engine]: https://docs.codeclimate.com/docs/excluding-files-and-folders#section-exclude-paths-for-specific-engines