Create pull_request_template.md

.github/pull_request_template.md

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+### Don't edit index.md directly!
+
+### To sign, create a file in `_data/signed/` named `<username>.yaml` with the following content:
+
+```yaml
+name: <your name here>
+link: <link to your profile or site>
+```
+
+### Example
+
+```yaml
+name: Richard Matthew Stallman
+link: https://stallman.org/
+```