feat: Create pull_request_template.md (slsa-framework#3268)

Adds a PR template as default PR description:

Hopefully this can also remind us to to keep docs up-to-date, as well,
like in slsa-framework#3263

---------

Signed-off-by: Ramon Petgrave <32398091+ramonpetgrave64@users.noreply.github.com>
Signed-off-by: Ramon Petgrave <ramon.petgrave64@gmail.com>
Co-authored-by: Ian Lewis <ianlewis@google.com>
Signed-off-by: Ramon Petgrave <ramon.petgrave64@gmail.com>

.github/pull_request_template.md

@@ -0,0 +1,15 @@
+# Summary
+
+...
+
+## Testing Process
+
+...
+
+## Checklist
+
+- [ ] Review the contributing [guidelines](./../CONTRIBUTING.md)
+- [ ] Add a reference to related issues in the PR description.
+- [ ] Update documentation if applicable.
+- [ ] Add unit tests if applicable.
+- [ ] Add changes to the [CHANGELOG](./../CHANGELOG.md) if applicable.