MAGETWO-65766: Create Pull Request Template

Author: Oleksii Korshenko

PULL_REQUEST_TEMPLATE.md

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+<!--- Provide a general summary of the Pull Request in the Title above -->
+
+### Description
+<!--- Provide a description of the changes proposed in the pull request -->
+
+### Fixed Issues (if relevant)
+<!--- Provide a list of fixed issues in the format magento/magetno2#<issue_number>, if relevant  -->
+1. magento/magetno2#<issue_number>: Issue title
+2. ...
+
+### Manual testing scenarios
+<!--- Provide a set of unambiguous steps to test the proposed code change -->
+1. ...
+2. ...
+
+### Contribution checklist
+ - [ ] Pull request has a meaningful description of its purpose
+ - [ ] All commits are accompanied by meaningful commit messages
+ - [ ] All new or changed code is covered with unit/integration tests (if applicable)
+ - [ ] All automated tests passed successfully (all builds on Travis CI are green)