Add PR template

Add pull request template to speedup communication and save maintainers' efforts.

PULL_REQUEST_TEMPLATE.md

+**Purpose of proposed changes**
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+Please give us an idea about why these changes should be applied. If actual PR fixes some opened issues, please reference them here.
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+**Essential steps taken**
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+If possible, please highlight most essential parts of your solution in order to speed up our understanding of changes made.