Add CODEOWNERS file (temporalio#107)

.github/CODEOWNERS

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+# Syntax is here:
+# https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners#codeowners-syntax
+
+*                @temporalio/core

.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**
+