[docs] PR template to include commit message

CONTRIBUTING.md

@@ -97,6 +97,8 @@ versioning guidelines:
   colon. Examples:
   * "docs: fix grammar error"
   * "http conn man: add new feature"
+* Your PR commit message will be used as the commit message when your PR is merged. You should 
+  update this field if your PR diverges during review.
 * Your PR description should have details on what the PR does. If it fixes an existing issue it
   should end with "Fixes #XXX".
 * When all of the tests are passing and all other conditions described herein are satisfied, a
@@ -190,10 +192,12 @@ and false.
   organization specific shortcuts into the code.
 * If there is a question on who should review a PR please discuss in Slack.
 * Anyone is welcome to review any PR that they want, whether they are a maintainer or not.
+* Please make sure that the PR title, commit message, and description are updated if the PR changes 
+  significantly during review.
 * Please **clean up the title and body** before merging. By default, GitHub fills the squash merge
   title with the original title, and the commit body with every individual commit from the PR.
   The maintainer doing the merge should make sure the title follows the guidelines above and should
-  overwrite the body with the original extended description from the PR (cleaning it up if necessary)
+  overwrite the body with the original commit message from the PR (cleaning it up if necessary)
   while preserving the PR author's final DCO sign-off.
 * If a PR includes a deprecation/breaking change, notification should be sent to the
   [envoy-announce](https://groups.google.com/forum/#!forum/envoy-announce) email list.

PULL_REQUESTS.md

@@ -12,13 +12,21 @@ explaining the overall change. Both the component and the explanation must be lo
 * router:add x-envoy-overloaded header
 * tls: add support for specifying TLS session ticket keys
 
-### <a name="desc"></a>Description
+### <a name="desc"></a>Commit Message
 
-The description field should include a more verbose explanation of what this PR
-does. If this PR causes a change in behavior it should document the behavior
-before and after   If fixing a bug, please describe what the original issue is and
-how the change resolves it. If it is configuration controlled, it should note
-how the feature is enabled etc...
+The commit message field should include an explanation of what this PR
+does. This will be used as the final commit message that maintainers will use to
+populate the commit message when merging. If this PR causes a change in behavior
+it should document the behavior before and after. If fixing a bug, please
+describe what the original issue is and how the change resolves it. If it is
+configuration controlled, it should note how the feature is enabled etc...
+
+
+### <a name="desc"></a>Additional Description
+
+The additional description field should include information of what this PR does
+that may be out of scope for a commit message. This could include additional
+information or context useful to reviewers.
 
 ### <a name="risk"></a>Risk
 

PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,8 @@
 For an explanation of how to fill out the fields, please see the relevant section
 in [PULL_REQUESTS.md](https://github.com/envoyproxy/envoy/blob/master/PULL_REQUESTS.md)
 
-Description:
+Commit Message:
+Additional Description:
 Risk Level:
 Testing:
 Docs Changes: