Temporary information for 3rd party pull requests (#21)

.github/PULL_REQUEST_TEMPLATE

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+Thank you for considering a contribution!
+
+Please be aware that we currently won't merge 3rd party PRs because this repository
+is temporary. We are waiting for the decision of the OpenTelemetry technical
+commitee on the donation of this repository.
+
+In case the donation gets accepted, this repository will move to the [GitHub open-telemetry organization](https://github.com/open-telemetry),
+which requires signing a different CLA. At that point we will start working on reviewing and merging 3rd party PRs.