chore: add PULL_REQUEST_TEMPLATE.md (dequelabs#29)

Author: stephenmathieson

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+<< Describe the changes >>
+
+Closes issue:
+
+## Reviewer checks
+
+**Required fields, to be filled out by PR reviewer(s)**
+- [ ] Follows the commit message policy, appropriate for next version
+- [ ] Has documentation updated, a DU ticket, or requires no documentation change
+- [ ] Includes new tests, or was unnecessary
+- [ ] Code is reviewed for security by: << Name here >>