Add pull request template.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+---
+name: Pull request
+about: Improve pandoc's code
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Before you continue, please make sure that you have
+
+- [ ] read the [contributing guidelines](https://github.com/jgm/pandoc/CONTRIBUTING.md)
+- [ ] searched [the issue tracker](https://github.com/jgm/pandoc/issues) for similar issues (the closed issues too).
+
+**What issue does this pull request address?**
+Fixes #0000 <!-- remove if not applicable -->
+
+**Describe the changes.**
+