.github: add pull request and issue template

.github/ISSUE_TEMPLATE.md

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+# Bug reporting
+
+A good bug report has some very specific qualities, so please read over our short document on
+[reporting bugs][report_bugs] before you submit your bug report. 
+
+To ask a question, go ahead and ignore this.
+
+[report_bugs]: ../Documentation/reporting_bugs.md

.github/PULL_REQUEST_TEMPLATE.md

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+# Contributing guidelines
+
+Please read our [contribution workflow][contributing] before submitting a pull request.
+
+[contributing]: ../CONTRIBUTING.md#contribution-flow