Adding CONTRIBUTING guide (#2087)

* Adding a CONTRIBUTING guide to help new users

* Fixing trailing whitespaces :/

* Remove cosmetic question on CONTRIBUTING

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+## How to contribute to Decidim
+
+We're really glad you're reading this, because we need volunteer developers to help this project come to fruition.
+
+If you haven't already, come find us in [Gitter](https://gitter.im/decidim/decidim). We want you working on things you're excited about.
+
+#### **Did you find a bug?**
+
+* **Do not open up a GitHub issue if the bug is a security vulnerability in Decidim**, and instead send us an email to security [at] decidim.org.
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/decidim/decidim/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/decidim/decidim/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+#### **Did you write a patch that fixes a bug?**
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+
+#### **Do you intend to add a new feature or change an existing one?**
+
+* Suggest your change in [Metadecidim](https://meta.decidim.barcelona/processes/roadmap) and start writing code.
+
+* Do not open an issue on GitHub until you have collected positive feedback about the change. GitHub issues are primarily intended for bug reports, fixes and already discussed features on [Metadecidim](https://meta.decidim.barcelona/processes/roadmap).
+
+#### **Do you have questions about the source code?**
+
+* Ask any question about how to use Decidim in [Gitter](https://gitter.im/decidim/decidim).
+
+#### **Do you want to contribute to Decidim documentation?**
+
+* Watch out the [Decidim Documentation repository](https://github.com/decidim/docs.decidim.org) and the [decidim.org](https://github.com/decidim/decidim.org) code website.
+
+#### **Do you want to contribute to Decidim design?**
+
+* Go to the [Decidim design repository](https://github.com/decidim/design) and propose the changes that you want.
+
+#### **Do you want to improve or add a new language for Decidim?**
+
+* Add a new issue asking for adding a user to the Crowdin project ([example issue](https://github.com/decidim/decidim/issues/2073)).
+
+Thanks! :heart: :heart: :heart:
+
+Decidim Team