docs: Add basic PR template

Signed-off-by: Terry Howe <tlhowe@amazon.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,6 @@
+**What this PR does / why we need it**:
+
+**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
+Fixes #
+
+<!-- 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. -->