CONTRIBUTING.md: Add contributing guide

docs/CONTRIBUTING.md

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+# Contributing
+
+Several ways you can contribute to this project.
+
+## Share button maps
+
+#### 1. Check out existing button maps
+
+You can see the existing button maps here:
+
+* [XML button maps](https://github.com/xbmc/peripheral.joystick/tree/master/peripheral.joystick/resources/buttonmaps/xml)
+
+#### 2. Create the button map
+
+Button maps can be generated from within the GUI. See the following wiki articles:
+
+* **[HOW-TO: Configure controllers](https://kodi.wiki/view/HOW-TO:Configure_controllers)**
+* **[HOW-TO: Map multiple controllers](https://kodi.wiki/view/HOW-TO:Map_multiple_controllers)**
+
+These map GUI elements to XML files. The XML files can be found in [userdata](https://kodi.wiki/view/Userdata):
+
+```
+userdata/addon_data/peripheral.joystick/resources/buttonmaps/xml/<joystick driver>/
+```
+
+#### 3. Submit the button map
+
+Fork [peripheral.joystick](https://github.com/xbmc/peripheral.joystick) and clone it to your working directory. Copy the XML file created by the GUI (or by hand) to the folder:
+
+```
+peripheral.joystick/resources/buttonmaps/xml/<joystick driver>/
+```
+
+Commit the file and send the pull request to the [peripheral.joystick](https://github.com/xbmc/peripheral.joystick) repo.
+
+## Create controller profiles
+
+If you want a new awesome controller to be shown in the GUI, you can create one yourself.
+
+#### 1. Check out existing controller profiles
+
+The controllers in the GUI are defined by Kodi add-ons. You can see the current set of add-ons in the [controller topology project](https://github.com/kodi-game/controller-topology-project):
+
+* [Kodi controller add-ons](https://github.com/kodi-game/controller-topology-project/tree/master/addons)
+
+#### 2. Create a new controller profile
+
+Controllers can be added by creating a controller add-on. The format is defined here:
+
+* [Controller add-on format](https://github.com/kodi-game/controller-topology-project/blob/master/Readme-Addons.md)