Add default PR template (#640)

* Add default PR template

* Update .github/PULL_REQUEST_TEMPLATE.md
Co-Authored-By: per1234 <accounts@perglass.com>
Co-authored-by: per1234 <accounts@perglass.com>

.github/PULL_REQUEST_TEMPLATE.md

+**Please check if the PR fulfills these requirements**
+- [ ] The PR has no duplicates (please search among the [Pull Requests](https://github.com/arduino/arduino-cli/pulls) before creating one)
+- [ ] The PR follows [our contributing guidelines](https://arduino.github.io/arduino-cli/CONTRIBUTING/#pull-requests)
+- [ ] Tests for the changes have been added (for bug fixes / features)
+- [ ] Docs have been added / updated (for bug fixes / features)
+
+
+* **What kind of change does this PR introduce?**
+<!-- Bug fix, feature, docs update, ... -->
+
+
+* **What is the current behavior?**
+<!-- You can also link to an open issue here -->
+
+
+* **What is the new behavior?**
+<!-- if this is a feature change -->
+
+
+
+* **Does this PR introduce a breaking change?**
+<!-- What changes might users need to make in their workflow or application due to this PR? -->
+
+
+
+* **Other information**:
+<!-- Any additional information that could help the review process -->
+
+
+---
+See [how to contribute](https://arduino.github.io/arduino-cli/CONTRIBUTING/)