Add template of pull request for contributors

[skip ci]

PULL_REQUEST_TEMPLATE.md

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+Please ensure your pull request adheres to the following guidelines: 
+
+- [ ] Please explain your motives to contribute this change: what problem you are trying to fix, what improvement you are trying to make
+- [ ] Use the following formatting style: [SonarSource/sonar-developer-toolset](https://github.com/SonarSource/sonar-developer-toolset#code-style)
+- [ ] Provide a unit test for any code you changed
+- [ ] If there is a [JIRA](http://jira.sonarsource.com/browse/SONAR) ticket available, please make your commits and pull request start with the ticket ID (SONAR-XXXX)