Add PullApprove checks

Add PullApprove: https://pullapprove.com/opencontainers/runtime-spec/

Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

.pullapprove.yml

@@ -0,0 +1,9 @@
+approve_by_comment: true
+approve_regex: ^LGTM
+reject_regex: ^Rejected
+reset_on_push: true
+reviewers:
+  teams:
+  - runtime-spec-maintainers
+  name: default
+  required: 2