Add a basic PR template (#78)

Author: neonichu

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,5 @@
+_[Put a one line description of your change into the PR title, please be specific]_
+
+_[Explain the context, and why you're making that change. What is the problem you're trying to solve.]_
+
+_[Tests can be run by commenting `@swift-ci` test on the pull request, for more information see [this](README.md#testing)]_