Add Feature request template (#3403)

* Added feature request template, from wagtail's example which I feel is comprehensive enough (wagtail/wagtail#5596)

Signed-off-by: xlanor <tanjingkai@me.com>

* Update as per request to change label

Signed-off-by: xlanor <tanjingkai@me.com>

.github/FEATURE_REQUEST.md

@@ -0,0 +1,43 @@
+---
+name: "Feature Request"
+about: Suggest an idea for improving Thanos
+title: ''
+labels: "feature request / improvement"
+assignees: ''
+
+---
+
+### Is your proposal related to a problem?
+
+<!--
+  Provide a clear and concise description of what the problem is.
+  For example, "I'm always frustrated when..."
+  Alternatively, an issue can also be linked here.
+-->
+
+(Write your answer here.)
+
+### Describe the solution you'd like
+
+<!--
+  Provide a clear and concise description of what you want to happen.
+-->
+
+(Describe your proposed solution here.)
+
+### Describe alternatives you've considered
+
+<!--
+  Let us know about other solutions you've tried or researched.
+-->
+
+(Write your answer here.)
+
+### Additional context
+
+<!--
+  Is there anything else you can add about the proposal?
+  You might want to link to related issues here, if you haven't already.
+-->
+
+(Write your answer here.)