Add Contributing guide

misc/CONTRIBUTING.md

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+# Contributing Guide
+
+## Pull Requests
+
+If you are familiar you can add a *section* or a *service* yourself via **pull request**. This is the better way.
+
+If you are not familiar with Github and PRs you can open a new issue and explain what you want to add there.
+
+### Adding a new section
+To add a new section copy the following template and complete with the information:
+
+```
+## Section Title
+
+<img width="16" src="misc/forbidden.png"> </img> **Avoid**:
+[OPTIONAL EXPLANATION OF WHY THESE SERVICES ARE BAD]
+* Bad Service 1
+* Bad Service 2
+* Bad Service 3
+
+<img width="16" src="misc/check.png"> </img>  **Instead use:**
+[OPTIONAL EXPLANATION OF WHY THESE SERVICES ARE GOOD]
+* [Good Service 1](https://link-to-source-or-page1) - Little description of the service
+* [Good Service 2](https://link-to-source-or-page2) - Little description of the service
+* [Good Service 2](https://link-to-source-or-page3) - Little description of the service
+```
+You also need to add the section to the **Table of Contents (Index)** at the top of the document:
+
+`* [Section Title](#section-title)`
+
+### Adding a new service
+
+To add a new service just take the following template and add it to the section where it belongs:
+
+`* [Service Name](https://link-to-source-or-page) - Little description of the service`
+
+> You can add either a *good* (use) or a *bad* (avoid) service.