root: Add PULL_REQUEST_TEMPLATE.md

PULL_REQUEST_TEMPLATE.md

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+Problem
+
+Explain the context and why you're making that change.  What is the
+problem you're trying to solve? In some cases there is not a problem
+and this can be thought of being the motivation for your change.
+
+Solution
+
+Describe the modifications you've done.
+
+Result
+
+What will change as a result of your pull request? Note that sometimes
+this section is unnecessary because it is self-explanatory based on
+the solution.