[dev] PIP 27: Add checklist in github pull request template (#3252)

*Motivation*

With the increase of contributions, more and more features are
added pretty quickly. However these features are not documented.
And there is no checklist for contributors to check when
contributing a feature.

*Changes*

This PIP is proposing to improve the github pull request template
by adding a checklist for contributors to check. It also help
improve the review process.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,12 +1,74 @@
+<--
+### Contribution Checklist
+
+  - Name the pull request in the form "[Issue XYZ][component] Title of the pull request", where *XYZ* should be replaced by the actual issue number.
+    Skip *Issue XYZ* if there is no associated github issue for this pull request.
+    Skip *component* if you are unsure about which is the best component. E.g. `[docs] Fix typo in produce method`.
+
+  - 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.
+
+**(The sections below can be removed for hotfixes of typos)**
+-->
+
+*(If this PR fixes a github issue, please add `Fixes #<xyz>`.)*
+
+Fixes #<xyz>
+
+*(or if this PR is one task of a github issue, please add `Master Issue: #<xyz>` to link to the master 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.
+
+*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.
+*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*
+
+### Does this pull request potentially affect one of the following parts:
+
+*If `yes` was chosen, please highlight the changes*
+
+  - Dependencies (does it add or upgrade a dependency): (yes / no)
+  - The public API: (yes / no)
+  - The schema: (yes / no / don't know)
+  - The default values of configurations: (yes / no)
+  - The wire protocol: (yes / no)
+  - The rest endpoints: (yes / no)
+  - The admin cli options: (yes / no)
+  - Anything that affects deployment: (yes / no / don't know)
 
-### Result
+### Documentation
 
-After your change, what will change.
+  - Does this pull request introduce a new feature? (yes / no)
+  - If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
+  - If a feature is not applicable for documentation, explain why?
+  - If a feature is not documented yet in this PR, please create a followup issue for adding the documentation