Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+Before submitting your PR, check that you've:
+
+- [ ] Bumped the version number
+- [ ] Documented new APIs with [Haddock markup](https://www.haskell.org/haddock/doc/html/index.html)
+- [ ] Added [`@since` declarations](http://haskell-haddock.readthedocs.io/en/latest/markup.html#since) to the Haddock
+
+After submitting your PR:
+
+- [ ] Update the Changelog.md file with a link to your PR
+- [ ] Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)
+
+<!---Thanks so much for contributing! :)
+
+_If these checkboxes don't apply to your PR, you can delete them_
+
+If you're unsure on what the new version number should be, feel free to ask.
+
+-->