Improve Chronology diagram management

Adding `Chronology diagram` on:
- Syntax
- Sub-diagram
- Readme

README.md

@@ -56,6 +56,7 @@ PlantUML is a component that allows you to create various UML diagrams through s
   - [SDL (Specification and Description Language)](http://plantuml.com/activity-diagram-beta#sdl)
   - [Ditaa diagram](http://plantuml.com/ditaa)
   - [Gantt diagram](http://plantuml.com/gantt-diagram)
+  - [Chronology diagram](http://plantuml.com/chronology-diagram)
   - [MindMap diagram](http://plantuml.com/mindmap-diagram)
   - [WBS (Work Breakdown Structure)](http://plantuml.com/wbs-diagram)
   - [Mathematical Notations (AsciiMath, JLaTeXMath)](http://plantuml.com/ascii-math)

src/net/sourceforge/plantuml/EmbeddedDiagram.java

@@ -119,6 +119,9 @@ public class EmbeddedDiagram extends AbstractTextBlock implements Line, Atom {
 		if (s.equals(EMBEDDED_START + "files"))
 			return "files";
 
+		if (s.equals(EMBEDDED_START + "chronology"))
+			return "chronology";
+
 		return null;
 	}
 

src/net/sourceforge/plantuml/syntax/LanguageDescriptor.java

@@ -123,6 +123,7 @@ public class LanguageDescriptor {
 		keyword.add("@startebnf");
 		keyword.add("@startregex");
 		keyword.add("@startfiles");
+		keyword.add("@startchronology");
 		keyword.add("@endwire");
 		keyword.add("@endbpm");
 		keyword.add("@enduml");
@@ -150,6 +151,7 @@ public class LanguageDescriptor {
 		keyword.add("@endebnf");
 		keyword.add("@endregex");
 		keyword.add("@endfiles");
+		keyword.add("@endchronology");
 		keyword.add("as");
 		keyword.add("also");
 		keyword.add("autonumber");