Create pull_request_template.md (skypilot-org#1501)

This basic template ensures tests are run and documented, while requiring minimal effort to fill in.

Author: concretevitamin

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+<!-- Describe the changes in this PR -->
+
+Describe the changes in this PR:
+
+
+<!-- Describe tests ran in a "Tested:" section -->
+<!-- Unit tests (tests/test_*.py) are part of Github CI; the below are additional tests. -->
+
+Tested (run the relevant ones):
+
+- [ ] Any manual or new tests for this the PR
+- [ ] All smoke tests: `bash tests/run_smoke_tests.sh` 
+- [ ] Relevant individual smoke tests: `bash tests/run_smoke_tests.sh test_fill_in_the_name` 
+- [ ] Backward compatibility tests: `bash tests/backward_comaptibility_tests.sh`