Add PR template (#100)

.github/PULL_REQUEST_TEMPLATE.md

+* **What kind of change does this PR introduce?** (Bug fix, feature, docs update, ...)
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+* **What is the current behavior?** (You can also link to an open issue here)
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+* **What is the new behavior (if this is a feature change)?**
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+* **Does this PR introduce a breaking change?** (What changes might users need to make in their application due to this PR?)
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+* **Other information**: