Create pull_request_template.md

Adding a basic PR template to at least remind myself to test in multiple browsers.

.github/pull_request_template.md

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+# Summary
+
+Fixes (issue number)
+
+Enter a summary of the changes made, here.
+
+# Testing
+
+Enter the browser versions this change was tested on below.
+
+| browser | version  |
+| ------- | -------- |
+| firefox | untested |
+| chrome  | untested |
+| safari  | untested |
+| edge    | untested |