Add CONTRIBUTING document

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to KrakenD, there are several ways
+you can contribute and make this project more awesome, please see below:
+
+## Reporting an Issue
+
+If you believe you have found an issue with the code please do not hesitate to file an issue in [Github](https://github.com/devopsfaith/krakend/issues). When
+filing the issue please describe the problem with the maximum level of detail
+and the steps to reproduce the problem, including information about your
+environment.
+
+You can also open an issue requesting for help or doing a question and it's
+also a good way of contributing since other users might be in a similar
+position.
+
+Please note we have a code of conduct, please follow it in all your interactions with the project.
+
+## Code Contributions
+
+When contributing to this repository, it is generally a good idea to discuss
+the change with the owners before investing a lot of time coding. The process
+could be:
+
+1. Open an issue explaining the improvment or fix you want to add
+2. [Fork the project](https://github.com/devopsfaith/krakend/fork_select)
+3. Code it in your fork
+4. Submit a [pull request](https://help.github.com/articles/creating-a-pull-request) referencing the issue
+
+
+Your work will then be reviewed as soon as possible (suggestions about some
+changes, improvements or alternatives may be given).
+
+**Don't forget to add tests**, make sure that they all pass!
+
+# Help with Git
+
+Once the repository is forked, you should track the upstream (original) one
+using the following command:
+
+    git remote add upstream https://github.com/devopsfaith/krakend.git
+
+Then you should create your own branch:
+
+    git checkout -b <prefix>/<micro-title>-<issue-number>
+
+Once your changes are done (`git commit -am '<descriptive-message>'`), get the
+upstream changes:
+
+    git checkout master
+    git pull --rebase origin master
+    git pull --rebase upstream master
+    git checkout <your-branch>
+    git rebase master
+
+Finally, publish your changes:
+
+    git push -f origin <your-branch>
+
+You should be now ready to make a pull request.