[GitHub] Add pull request template (sonic-net#151)

Add GitHub pull request template

.github/pull_request_template.md

@@ -0,0 +1,22 @@
+<!-- Provide a general summary of your changes in the Title above -->
+
+#### Description
+<!--
+     Describe your changes in detail
+-->
+
+#### Motivation and Context
+<!--
+     Why is this change required? What problem does it solve?
+     If this pull request closes/resolves an open Issue, make sure you
+     include the text "fixes #xxxx", "closes #xxxx" or "resolves #xxxx" here
+-->
+
+#### How Has This Been Tested?
+<!--
+     Please describe in detail how you tested your changes.
+     Include details of your testing environment, and the tests you ran to
+     see how your change affects other areas of the code, etc.
+-->
+
+#### Additional Information (Optional)