Merge pull request #1478 from openark/pr-template-unmaintained

Clarify the repo is not maintained, in PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

+# I am taking time off from maintaining this repo. I will not be reviewing pull requests.
+# Details in https://code.openark.org/blog/mysql/reducing-my-oss-involvement-and-how-it-affects-orchestrator-gh-ost
+
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