.github: add feature request template

.github/ISSUE_TEMPLATE/feature_request.md

+---
+name: Feature Request
+about: Ask for missing features or improvements
+
+---
+
+#### Description
+<!-- Please describe your use case, why you need this feature and why this
+feature is important for RIOT. -->
+
+### Useful links
+<!-- Please include links to any documentation that you think is useful. -->
+
+<!-- Thanks for contributing! -->