add details in issue template (#2085)

indirectly request more details

.github/ISSUE_TEMPLATE.md

@@ -3,11 +3,47 @@
   - Please provide source code and commit sha if you found a bug.
   - Review existing issues and provide feedback or react to them.
 
-- go version:
-- gin version (or commit ref):
-- operating system:
-
 ## Description
 
-## Screenshots
+<!-- Description of a problem -->
+
+## How to reproduce
+
+<!-- The smallest possible code example to show the problem that can be compiled, like -->
+```
+package main
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	g := gin.Default()
+	g.GET("/hello/:name", func(c *gin.Context) {
+		c.String(200, "Hello %s", c.Param("name"))
+	})
+	g.Run(":9000")
+}
+```
 
+## Expectations
+
+<!-- Your expectation result of 'curl' command, like -->
+```
+$ curl http://localhost:8201/hello/world
+Hello world
+```
+
+## Actual result
+
+<!-- Actual result showing the problem -->
+```
+$ curl -i http://localhost:8201/hello/world
+<YOUR RESULT>
+```
+
+## Environment
+
+- go version:
+- gin version (or commit ref):
+- operating system: