Add FR template (#6296)

Follow-up to <https://github.com/koreader/koreader/pull/6291>.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,20 @@
+---
+name: Feature request
+about: Share an idea or suggestion
+title: 'FR: [a handful of words describing the FR]'
+labels: enhancement
+assignees: ''
+
+---
+
+**Does your feature request involve difficulty completing a task? Please describe.**
+A clear and concise description of what the problem is. Ex. I think it takes too many steps to [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you'd like to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any additional context or screenshots about the feature request here.