adds english contributing guide

CONTRIBUTING_EN.md

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+## Team members
+
+* Raimond Garcia [github](https://github.com/voodoorai2000) | [twitter](https://twitter.com/voodoorai2000)
+* Juanjo Bazán [github](https://github.com/xuanxu) | [twitter](https://twitter.com/xuanxu)
+* Enrique García Cota [github](https://github.com/kikito) | [twitter](https://twitter.com/otikik)
+* Alberto Garcia Cabeza [github](https://github.com/decabeza) | [twitter](https://twitter.com/decabeza)
+
+## Report an issue
+
+The prefered way to report any bug is [opening an issue in the project's Github repo](https://github.com/AyuntamientoMadrid/participacion/issues/new).
+
+For more informal communication, contact team members via twitter
+
+## Resolve an issue
+
+Admins tag issues using two label related with collaboration availability:
+
+* `PRs-welcome`: [issues labeled with PRs-welcome](https://github.com/AyuntamientoMadrid/participacion/labels/PRs-welcome) are well defined features ready to be implemented by whoever wants to do it.
+
+* `Not-ready`: with this label admins mark features or changes that are not well defined yet or subject to an internal decision. Is not a good idea to start implementation of these isuues.
+
+If you want to contribute code to solve an issue:
+
+* Add a comment to tell everyone you are working on the issue.
+* If an issue has someone assigned it means that person is already working on it.
+* Fork the project.
+* Create a topic branch based on master.
+* Commit there your code to solve the issue.
+* Make sure all test are passing (and add specs to test any new feature if needed).
+* Open a *pull request* to the main repository describing what issue you are addressing.
+
+## Other ways of contributing without coding
+
+* If you think there's a feature missing, or find a bug, create an issue (make sure it has not already been reported).
+* You can also help promoting the project talking about it in your social networks.
+
+## How to report an issue
+
+* Try to use a descriptive and to-the-point title
+* Is a good idea to include some of there sections:
+  * Steps to reproduce the bug
+  * Expected behaviour/response
+  * Actual response
+* Sometimes it is also helpful if you mention your operating system, browser version and installed plugins.
+
+Thanks! :heart: :heart: :heart: