Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+### Contribution Checklist
+
+  - PR title format should be *[type][component] summary*. For details, see *[Guideline - Pulsar PR Naming Convention](https://docs.google.com/document/d/1d8Pw6ZbWk-_pCKdOmdvx9rnhPiyuxwq60_TrD68d7BA/edit#heading=h.trs9rsex3xom)*. 
+
+  - Fill out the template below to describe the changes contributed by the pull request. That will give reviewers the context they need to do the review.
+
+  - Each pull request should address only one issue, not mix up code from multiple issues.
+
+  - Each commit in the pull request has a meaningful commit message
+
+  - Once all items of the checklist are addressed, remove the above text and this checklist, leaving only the filled out template below.
+-->
+
+<!-- Either this PR fixes an issue, -->
+
+Fixes #<xyz>
+
+<!-- or this PR is one task of an issue -->
+
+Master Issue: #<xyz>
+
+### Motivation
+
+<!-- Explain here the context, and why you're making that change. What is the problem you're trying to solve. -->
+
+### Modifications
+
+<!-- Describe the modifications you've done. -->
+
+### Verifying this change
+
+- [ ] Make sure that the change passes the CI checks.
+
+*(Please pick either of the following options)*
+
+This change is a trivial rework / code cleanup without any test coverage.
+
+*(or)*
+
+This change is already covered by existing tests, such as *(please describe tests)*.
+
+*(or)*
+
+This change added tests and can be verified as follows:
+
+*(example:)*
+  - *Added integration tests for end-to-end deployment with large payloads (10MB)*
+  - *Extended integration test for recovery after broker failure*
+
+### Documentation
+
+<!-- DO NOT REMOVE THIS SECTION. CHECK THE PROPER BOX ONLY. -->
+
+- [ ] `doc-required` 
+(Your PR needs to update docs and you will update later)
+
+- [ ] `doc-not-needed` 
+(Please explain why)
+
+- [ ] `doc` 
+(Your PR contains doc changes)
+
+- [ ] `doc-complete`
+(Docs have been already added)