Add a PR template

Author: bbatsov

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+**Replace this placeholder text with a summary of the changes in your PR.
+The more detailed you are, the better.**
+
+-----------------
+
+Before submitting a PR make sure the following things have been done (and denote this
+by checking the relevant checkboxes):
+
+- [ ] The commits are consistent with our [contribution guidelines](../blob/master/CONTRIBUTING.md)
+- [ ] The new code is not generating bytecode or `M-x checkdoc` warnings
+- [ ] You've updated the [changelog](../blob/master/CHANGELOG.md) (if adding/changing user-visible functionality)
+- [ ] You've updated the readme (if adding/changing user-visible functionality)
+
+Thanks!