add pull request template

Author: cs01

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+[] I have added an entry to `docs/changelog.md`
+
+<!-- provide evidence of testing, preferably with command(s) that can be copy+pasted by others -->
+
+## Test plan
+Tested by running
+```
+# command(s) to exercise these changes
+```