[docs] add PR template to guide add-on developers

Author: MartijnKaijser

PULL_REQUEST_TEMPLATE.md

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+### Description
+<!--- Provide a short summary of submitted add-on in case it's a new addition. -->
+<!--- If it's plugin update only highlight biggest changes if needed. -->
+<!--- Make sure you follow the checklist below before finalising your pull-request. -->
+### Checklist:
+<!--- Go over all the following points, and put an `x` in all the boxes that apply like this: [X] -->
+<!--- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
+- [ ] My code follows the [add-on rules](http://kodi.wiki/view/Add-on_rules) and [piracy stance](http://kodi.wiki/view/Official:Forum_rules#Piracy_Policy) of this project. 
+- [ ] I have read the [CONTRIBUTING](https://github.com/xbmc/repo-scripts/blob/master/CONTRIBUTING.md) document
+- [ ] Each add-on submission should be a sinlge commit with using the following style: [sciprt.foo.bar] v1.0.0
+
+Addtional information :
+- Submitting your add-on to this specific branch makes it available to any Kodi version equal or higher than the branch name with the applicable Kodi dependencies limits.
+- [add-on development](http://kodi.wiki/view/Add-on_development) wiki page.
+- Kodi [pydocs](http://kodi.wiki/view/PyDocs) provide information about the Python API
+- [PEP8](https://www.python.org/dev/peps/pep-0008/) codingstyle which is considered best practise but not mandatory.
+- This add-on repository has automated code guideline check which could help you improve your coding. You can find the results of these check at [Codacy](https://www.codacy.com/app/Kodi/repo-scripts/dashboard). You can create your own account as well to continuesly monitor your python coding before submitting to repo.
+- Development questions can be asked in the [add-on development](http://forum.kodi.tv/forumdisplay.php?fid=26) section on the Kodi forum.