docs: add rocketchat service usage example

service/rocketchat/usage.md

@@ -0,0 +1,61 @@
+# RocketChat Usage
+
+Install notifier using:
+
+* `go get -u github.com/nikoksr/notify`
+
+
+## Steps to follow
+
+These are general and very high level instructions
+
+1. Login to rocketchat and create a personal token `Profile -> My Account -> Security -> Personal Access Tokens`
+2. Copy your *UserID and Token* for usage below
+3. Add the user to channels where you want to send message
+4. Note down *Channel Names* where you want to post a messages. Channel names are  *Case Sensitive*.
+5. Grab the URL for rocketchat server, for this example we are going to user `localhost` and `scheme` is http.
+6. Incase endpoint is exposed on a different port then default on localhost
+   you can input the serverURL with port i.e `localhost:3000`
+
+
+
+## Sample Code
+
+```go
+package main
+
+import (
+  "github.com/nikoksr/notify"
+  "github.com/nikoksr/notify/service/rocketchat"
+  "golang.org/x/net/context"
+)
+
+func main() {
+  // Assuming you already have a rocketchat personal token and userID
+  // Provide your server endpoint , protocol , userID  and token
+  rocketChatSvc, err := rocketchat.New("localhost", "http", "pLcfBy8zgFDYryQtG", "kNdevpAnDPxh3vwjGELFStFFOI0m0nU_AIN4B0BydtN")
+if err != nil {
+		panic(err)
+	}
+  // Add channel names where message is being sent
+  // Channel names are case sensitive
+  rocketChatSvc.AddReceivers("general", "Notify")
+
+  notifier := notify.New()
+
+  // Tell notifier to use the rocketchat service. You can repeat the above process
+  // for as many services as you like and just tell the notifier to use them.
+  notifier.UseServices(rocketChatSvc)
+
+  // Send a message
+  err := notifier.Send(
+    context.Background(),
+    "Welcome",
+    "I am a bot written in Go!",
+  )
+
+  if err != nil {
+    panic(err)
+  }
+}
+```