Add initial directory for Pipeline to TaskRun custom task 🎁

I had tried to add the custom task and OWNERS file all at once in
tektoncd#770 but @jerop pointed
out it's probably a better approach to just add the OWNERS file first,
which would allow getting the initial approval needed to kick off the
project and from then on the project OWNERS can take care of reviews
(and she documented this in
tektoncd#782 !)

So this commit adds just the OWNERS (more welcome if anyone else is
interested!) for the pipeline to taskrun custom task, and there will be
a follow-up PR with the initial controller logic.

Project proposal: tektoncd/community#447

pipeline-to-taskrun/OWNERS

@@ -0,0 +1,6 @@
+# The OWNERS file is used by prow to automatically merge approved PRs.
+
+approvers:
+- bobcatfish
+- jerop
+- wlynch

pipeline-to-taskrun/README.md

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+# Pipeline to TaskRun
+
+This project will be a controller that enables an experimental [custom task](https://github.com/tektoncd/pipeline/blob/main/docs/runs.md)
+that will allow you to execute a Pipeline (with [limited features](#supported-pipeline-features)) via a TaskRun, enabling you to
+run a Pipeline in a pod ([TEP-0044](https://github.com/tektoncd/community/blob/main/teps/0044-decouple-task-composition-from-scheduling.md)).