Added explanation of how to use the maven plugin

README.md

@@ -19,6 +19,27 @@ Astra has been developed and tested at Alfa to improve the velocity at which lar
 * Astra can be run as part of a Java application, using `astra-core` as a dependency and using the refactors it provides. For an illustration of how to do this, please see the [README in astra-core](./astra-core/README.md). The code can be found in [astra-example](./astra-example).
 * For cases needing a more bespoke approach, [astra-core](./astra-core/README.md) also provides an SPI for writing your own custom `ASTOperation`s. See the `astra-core` README for further details.
 * For very simple cases, there is also a command line interface which exposes a small subset of Astra's refactoring operations. Please see [astra-cli](./astra-cli/README.md) for more information.
+* Astra can also be run as a plugin during your build e.g. `mvn org.alfasoftware:astra-maven-plugin:2.4.2:refactor -Dastra.usecase=org.my.MyUseCase` though this requires `MyUseCase` to be available on your classpath. Therefore it is often better to amend your pom so that Astra can be used when needed and the UseCase can be retrieved from a separate library e.g.
+
+```
+      <build>
+        <plugins>
+          <plugin>
+            <groupId>org.alfasoftware</groupId>
+            <artifactId>astra-maven-plugin</artifactId>
+            <version>2.4.2</version>
+            <dependencies>
+              <dependency>
+                <groupId>org.yourcompany</groupId>
+                <artifactId>usecase-library</artifactId>
+                <version>1.0.0</version>
+              </dependency>
+            </dependencies>          
+          </plugin>  		
+        </plugins>
+      </build>
+```
+Then, as the plugin and version is defined, you can simply invoke `mvn astra:refactor -Dastra.usecase=org.my.MyUseCase`
 
 ## Why would I use Astra?
 A simple and common use case is renaming a method, and updating all the callers of that method so that they use the new name.