Updating README with SlackAttachment notes

Author: ashwanthkumar

README.md

@@ -21,21 +21,33 @@ libraryDependencies += "in.ashwanthkumar" % "slack-java-webhook" % "0.0.2"
 
 ## Usage
 ```java
-Slack slack = new Slack("https://hooks.slack.com/services/...");
-slack.icon(":smiling_imp:") // Anything from http://www.emoji-cheat-sheet.com/ should work here
+// Using SlackMessage
+new Slack().icon(":smiling_imp:") // Anything from http://www.emoji-cheat-sheet.com/ should work here
     .sendToUser("slackbot")
     .displayName("slack-java-client")
     .push(new SlackMessage("Text from my Slack-Java-Client"));
+
+// Using SlackAttachment
+new Slack()
+    .sendToUser("slackbot")
+    .displayName("slack-java-client")
+    .push(new SlackAttachment("Text from my Slack-Java-Client").author("ashwanthkumar", "https://avatars0.githubusercontent.com/u/600279?v=3&s=40"));
+
 ```
 
 ## Notes
-With `SlackMessage` you can create rich text like
+With `SlackMessage` you can create rich text as specified in https://api.slack.com/docs/formatting. Example usage
 ```java
-new SlackMessage()
-    .text("Some text can be ")
+new SlackMessage("Some text can be")
     .italic("Italic")
     .text(". :)")
 ```
+With `SlackAttachment` you can create much more sophisticated rich text as specified in https://api.slack.com/docs/attachments. Example usage
+```java
+new SlackAttachment()
+    .author("ashwanthkumar")
+    .author("ashwanthkumar", "https://avatars0.githubusercontent.com/u/600279?v=3&s=40")
+```
 
 Available methods
 - `text`