doc: add doc for Pipeline function

docs/function.md

@@ -29,6 +29,8 @@ import (
 -   [Compose](#Compose)
 -   [Debounced](#Debounced)
 -   [Delay](#Delay)
+-   [Pipeline](#Pipeline)
+-   [Schedule](#Schedule)
 -   [Watcher](#Watcher)
 
 <div STYLE="page-break-after: always;"></div>
@@ -298,6 +300,49 @@ func main() {
 }
 ```
 
+### <span id="Pipeline">Pipeline</span>
+
+<p>Pipeline takes a list of functions and returns a function whose param will be passed into
+the functions one by one.</p>
+
+<b>Signature:</b>
+
+```go
+func Pipeline(funcs ...func(interface{}) interface{}) func(interface{}) interface{}
+```
+
+<b>Example:</b>
+
+```go
+package main
+
+import (
+    "fmt"
+    "github.com/duke-git/lancet/function"
+)
+
+func main() {
+    addOne := func(x interface{}) interface{} {
+		return x.(int) + 1
+	}
+	double := func(x interface{}) interface{} {
+		return 2 * x.(int)
+	}
+	square := func(x interface{}) interface{} {
+		return x.(int) * x.(int)
+	}
+
+	f := function.Pipeline(addOne, double, square)
+
+    result := fn(2)
+
+    fmt.Println(result)
+
+    // Output:
+    // 36
+}
+```
+
 ### <span id="Watcher">Watcher</span>
 
 <p>Watcher is used for record code excution time. can start/stop/reset the watch timer. get the elapsed time of function execution.</p>