Add pull request template to remind us to update the changelog

Author: henrymercer

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!-- Thank you for submitting a pull request. Please read our pull request guidelines before
+  submitting your pull request:
+  https://github.com/github/vscode-codeql/blob/master/CONTRIBUTING.md#submitting-a-pull-request.
+-->
+
+Replace this with a description of the changes your pull request makes.
+
+## Checklist
+
+- [ ] This pull request contains no user-visible changes, or the [CHANGELOG.md](../extensions/ql-vscode/CHANGELOG.md) has been updated.