Add Contribute section to readme

Author: zborboa-g

README.md

@@ -223,3 +223,10 @@ MultiCurl::success($callback)
 MultiCurl::unsetHeader($key)
 MultiCurl::verbose($on = true)
 ```
+
+### Contribute
+1. Check for open issues or open a new issue to start a discussion around a bug or feature.
+1. Fork the repository on GitHub to start making your changes.
+1. Write one or more tests for the new feature or that expose the bug.
+1. Make code changes to implement the feature or fix the bug.
+1. Send a pull request to get your changes merged and published.