Add a minimal PR template (copy from hs-bindgen)

Author: dschrempf

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+---
+
+- [ ] I have updated the changelog. I have included a migration hint if this is a breaking change.
+
+- [ ] I have updated the manual.
+
+- [ ] I have removed all mentions of closed tickets in the code.
+
+- [ ] I have associated each new TODO item with a ticket, using the following syntax:
+
+```hs
+-- TODO <link to issue>
+-- Brief description
+```