GitHub: add feature request template (#10294)

Author: stsewd

.github/ISSUE_TEMPLATE/feature.md

@@ -0,0 +1,20 @@
+---
+name: Feature/improvement proposal
+about: Propose a new feauture or improvement for Read the Docs
+---
+
+## What's the problem this feature will solve?
+
+What are you trying to do, that you are unable to achieve with the current state of Read the Docs.
+
+## Describe the solution you'd like
+
+Clear and concise description of what you want to happen.
+
+## Alternative solutions
+
+Have you tried to workaround the problem? Or a different approach to solving this issue?
+
+## Additional context
+
+Add any other context, links, etc. relevant to the feature request.