Add PR template. (#1312)

Provides a PR template for github, asking PR submitters to also review a PR themselves (pretty please) and pointing to both generic PR review guidelines as well as the recently added review guidelines in the maintainer guide on the RViz wiki.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+<!--
+Thanks for submitting a Pull Request!
+
+Please shortly explain your contribution, and if fixing an issue from the tracker, please add a link to the issue.
+
+Be sure to go over each item in the list below before submitting your pull request.
+-->
+
+### Description
+
+Add your description here
+
+
+### Checklist
+
+- [ ] If you are changing RViz UI, then please include screenshots showing how things looked *before* and *after* your change
+- [ ] Did you change how RViz works? Added new functionality? Do not forget to update the tutorials and/or documentation on the [ROS wiki](http://wiki.ros.org/rviz)
+- [ ] If needed, please include (updated) tests with your PR
+- [ ] While waiting for someone to review your request, please consider reviewing [another open pull request](https://github.com/ros-visualization/rviz/pulls) to support the maintainers of RViz. Refer to the [ROS code review guide](https://github.com/rosin-project/ros_code_review_guide/blob/master/README.md) for general reviewing guidelines and [RViz Wiki: Reviewing Pull Requests](https://github.com/ros-visualization/rviz/wiki/Maintainer-Guide#reviewing-pull-requests) for RViz specific guidelines