Extensions readme (#50)

README.md

@@ -58,4 +58,76 @@ options.getSafelist().removeProtocols("img", "src", "http", "https");
 
 String html2 = Marked.marked("![alt text](/img/some-image.png \"title\")", options);
   // => <p><img src="/img/some-image.png" alt="alt text" title="title"></p>
+```
+
+## Extensions
+
+Markedj can be extended by implementing [custom extensions](https://github.com/gitbucket/markedj/blob/master/src/main/java/io/github/gitbucket/markedj/extension/Extension.java).
+Extensions can be used by adding them to the options.
+
+```java
+Options options = new Options();
+options.addExtension(new NotificationExtension());
+String html = Marked.marked("! This is an info message", options);
+  // => <div class="notification_info"><p>This is an info message</p></div>
+```
+
+### Notification extension
+
+The notification extension helps you to add information messages to your markdown content.
+Keep in mind, you still need the CSS to style the messages as desired.
+
+#### Info message
+```text
+! This is an info message
+```
+```html
+<div class="notification_info">
+  <p>This is an info message</p>
+</div>
+```
+
+#### Success message
+```text
+!v This is a success message
+```
+```html
+<div class="notification_success">
+  <p>This is a success message</p>
+</div>
+```
+
+#### Warning message
+```text
+!! This is a warning message
+```
+```html
+<div class="notification_warning">
+  <p>This is a warning message</p>
+</div>
+```
+
+#### Error message
+```text
+!x  This is an error message
+```
+```html
+<div class="notification_error">
+  <p>This is an error message</p>
+</div>
+```
+
+#### Multiline notifications
+Notifications can span multiple lines.
+
+```text
+! This is an info message
+! That spans over several lines
+```
+```html
+<div class="notification_info">
+<p>This is an info message
+That spans over several lines</p>
+</div>
+
 ```