Create PULL_REQUEST_TEMPLATE.md

PULL_REQUEST_TEMPLATE.md

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+Please use [PX4 Discuss](http://discuss.px4.io/) or [Slack](http://slack.px4.io/) to align on pull requests if necessary.
+
+**Test data / coverage**
+Logs uploaded to http://logs.px4.io or screenshots.
+
+**Describe problem solved by the proposed pull request**
+A clear and concise description of the problem, if any, this feature will solve. E.g. I'm always frustrated when ...
+
+**Describe your preferred solution**
+A clear and concise description of what you have implemented.
+
+**Describe possible alternatives**
+A clear and concise description of alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots for the feature request here.