[CELRun] Create cel folder and add OWNERS

Creating a folder for incubating CELRun project which will provide a
Common Expression Language Custom Task, as proposed in tektoncd/community#304
and adding the OWNERS to approve and review contributions to the project

cel/OWNERS

@@ -0,0 +1,15 @@
+# The OWNERS file is used by prow to automatically merge approved PRs.
+# See https://github.com/kubernetes/test-infra/tree/master/prow/plugins/approve/approvers.
+# It supports a list of approvers and reviewers and their github names must be provided.
+
+approvers:
+- bobcatfish
+- ImJasonH
+- jerop
+- pritidesai
+
+reviewers:
+- bobcatfish
+- ImJasonH
+- jerop
+- pritidesai