docs: add PR template (#70)

Resolves #66

Signed-off-by: Shiwei Zhang <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,12 @@
+### What this PR does / why we need it
+<!-- If this is a security issue, please do not discuss on GitHub. Please report any suspected or confirmed security issues directly to https://github.com/oras-project/oras/blob/main/OWNERS.md. -->
+
+### Which issue(s) this PR resolves / fixes
+<!-- Optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged. -->
+Resolves / Fixes #<issue_id>
+
+### Please check the following list
+- [ ] Does the affected code have corresponding tests, e.g. unit test, E2E test?
+- [ ] Does this change require a documentation update?
+- [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
+- [ ] Do all new files have an appropriate license header?