Add CODEOWNERS file (#863)

Author: taylanisikdemir

.github/CODEOWNERS

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+# This is a comment.
+# Each line is a file pattern followed by one or more owners.
+# Order is important; the last matching pattern takes the most
+# precedence.
+
+* @Groxx @abhishekj720 @dkrotx @taylanisikdemir @demirkayaender

.github/pull_request_template.md

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+<!-- Describe what has changed in this PR -->
+**What changed?**
+
+
+<!-- Tell your future self why have you made these changes -->
+**Why?**
+
+
+<!-- How have you verified this change? Tested locally? Added a unit test? Checked in staging env? -->
+**How did you test it?**
+
+
+<!-- Assuming the worst case, what can be broken when deploying this change to production? -->
+**Potential risks**
+
+<!-- Is it notable for release? e.g. schema updates, configuration or data migration required? If so, please mention it, and also update CHANGELOG.md -->
+**Release notes**
+
+<!-- Is there any documentation updates should be made for config, https://cadenceworkflow.io/docs/operation-guide/setup/ ? If so, please open an PR in https://github.com/uber/cadence-docs -->
+**Documentation Changes**