PR template: add point about Fixes: annotations

Author: wprzytula

.github/pull_request_template.md

@@ -11,3 +11,4 @@
 - [ ] PR description sums up the changes and reasons why they should be introduced.
 - [ ] I have implemented Rust unit tests for the features/changes introduced.
 - [ ] I have enabled appropriate tests in `Makefile` in `{SCYLLA,CASSANDRA}_(NO_VALGRIND_)TEST_FILTER`.
+- [ ] I added appropriate `Fixes:` annotations to PR description.