Added pull request template (#3443)

PULL_REQUEST_TEMPLATE.md

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+Please make sure you've read the [pull request](http://docs.mattermost.com/developer/contribution-guide.html#preparing-a-pull-request) section of our [code contribution guidelines](http://docs.mattermost.com/developer/contribution-guide.html).
+
+#### Summary
+Brief description of what this pull request does
+
+#### Unit Tests
+Have you updated the unit tests or added new ones to cover your changes? If not, you probably should
+
+#### Issue/Ticket Link
+Please link the GitHub issue or Jira ticket this PR addresses
+
+#### Has Critical Changes
+Does your PR touch critical sections of the codebase including, but not limited to, authentication, authorization or permissions?