document: add docs dir and middleware document (gin-gonic#1521)

* init docs dir

* add middleware document

* fix indent

* update docs

docs/how-to-build-an-effective-middleware.md

@@ -0,0 +1,137 @@
+# How to build one effective middleware?
+
+## Consitituent part
+
+The middleware has two parts:
+
+  - part one is what is executed once, when you initalize your middleware. That's where you set up all the global objects, logicals etc. Everything that happens one per application lifetime.
+
+  - part two is what executes on every request. For example, a database middleware you simply inject your "global" database object into the context. Once it's inside the context, you can retrieve it from within other middlewares and your handler furnction.
+
+```go
+func funcName(params string) gin.HandlerFunc {
+    // <---
+    // This is part one
+    // --->
+    // The follow code is an example
+    if err := check(params); err != nil {
+        panic(err)
+    }
+
+    return func(c *gin.Context) {
+        // <---
+        // This is part two
+        // --->
+        // The follow code is an example
+        c.Set("TestVar", params)
+        c.Next()    
+    }
+}
+```
+
+## Execution process
+
+Firstly, we have the follow example code:
+
+```go
+func main() {
+	router := gin.Default()
+
+	router.Use(globalMiddleware())
+
+	router.GET("/rest/n/api/*some", mid1(), mid2(), handler)
+
+	router.Run()
+}
+
+func globalMiddleware() gin.HandlerFunc {
+	fmt.Println("globalMiddleware...1")
+
+	return func(c *gin.Context) {
+		fmt.Println("globalMiddleware...2")
+		c.Next()
+		fmt.Println("globalMiddleware...3")
+	}
+}
+
+func handler(c *gin.Context) {
+	fmt.Println("exec handler.")
+}
+
+func mid1() gin.HandlerFunc {
+	fmt.Println("mid1...1")
+
+	return func(c *gin.Context) {
+
+		fmt.Println("mid1...2")
+		c.Next()
+		fmt.Println("mid1...3")
+	}
+}
+
+func mid2() gin.HandlerFunc {
+	fmt.Println("mid2...1")
+
+	return func(c *gin.Context) {
+		fmt.Println("mid2...2")
+		c.Next()
+		fmt.Println("mid2...3")
+	}
+}
+```
+
+According to [Consitituent part](#consitituent-part) said, when we run the gin process, **part one** will execute firstly and will print the follow information:
+
+```go
+globalMiddleware...1
+mid1...1
+mid2...1
+```
+
+And init order are:
+
+```go
+globalMiddleware...1
+    |
+    v
+mid1...1
+    |
+    v
+mid2...1
+```
+
+When we curl one request `curl -v localhost:8080/rest/n/api/some`, **part two** will execute their middleware and output the following information:
+
+```go
+globalMiddleware...2
+mid1...2
+mid2...2
+exec handler.
+mid2...3
+mid1...3
+globalMiddleware...3
+```
+
+In other words, run order are:
+
+```go
+globalMiddleware...2
+    |
+    v
+mid1...2
+    |
+    v
+mid2...2
+    |
+    v
+exec handler.
+    |
+    v
+mid2...3
+    |
+    v
+mid1...3
+    |
+    v
+globalMiddleware...3
+```