Create pull_request_template.md (#127)

Author: jrobble

.github/pull_request_template.md

@@ -0,0 +1,13 @@
+**Issues:**
+
+- https://github.com/openmpf/openmpf/issues/1 < Example
+- https://github.com/openmpf/openmpf/issues/2 < Example
+
+If a related issue doesn't exist, then create one first and assign it to yourself.
+
+**Related PRs:**
+
+- https://github.com/openmpf/openmpf/pull/22 < Example
+- https://github.com/openmpf/openmpf/pull/53 < Example
+
+Please review our [Contributor Guide](https://openmpf.github.io/docs/site/Contributor-Guide/index.html).