docs: add line service example usage

service/line/usage.md

@@ -0,0 +1,43 @@
+# Line Usage
+
+Install notifier using:
+
+* `go get -u github.com/nikoksr/notify`
+
+
+## Sample Code
+
+```go
+package main
+
+import (
+  "github.com/nikoksr/notify"
+  "github.com/nikoksr/notify/service/line"
+  "golang.org/x/net/context"
+)
+
+func main() {
+  // Assuming you already have a line messaging API credential
+  // Provide your channel secret and access token
+  lineService, _ := line.New("channelSecret", "channelAccessToken")
+
+  // Add id from various receivers
+  // You can try to use your own line id for testing
+  lineService.AddReceivers("userID1", "groupID1")
+
+  notifier := notify.New()
+  // Tell our notifier to use the line service. You can repeat the above process
+  // for as many services as you like and just tell the notifier to use them.
+  notifier.UseServices(lineService)
+
+  // Send a message
+  err := notifier.Send(context.Background(),
+    "Welcome",
+    "I am a bot written in Go!",
+  )
+
+  if err != nil {
+    panic(err)
+  }
+}
+```