Update README.md

Author: MenoData

README.md

@@ -13,3 +13,15 @@ This small library mainly serves for improvements of the JavaDoc of Time4J but m
 - **doctags.experimental** (documents the experimental status of a class)
 - **doctags.spec** (documents a specification requirement)
 - **doctags.exclude** (excludes a program element from public API and JavaDoc)
+
+Usage:
+------
+
+```java
+/** @doctags.concurrency {immutable} */
+```
+
+will be displayed like
+
+**Concurrency:** This class is immutable and can be used by multiple threads in parallel.
+