add usage guide to readme

Author: SirCotare

readme.md

@@ -2,6 +2,61 @@
 
 Checkstyle config for all JAVA projects.
 
+## Usage
+
+Add this repository as a submodule of your main project by creating a `.gitmodules` file:
+```
+[submodule "checkstyle-config"]
+	path = checkstyle-config
+	url = https://github.com/aboutbits/java-checkstyle-config
+	branch = main
+```
+
+Add the `maven-checkstyle-plugin` to your `pom.xml`:
+```xml
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-checkstyle-plugin</artifactId>
+    <version>3.5.0</version>
+    <configuration>
+        <configLocation>checkstyle-config/checkstyle.xml</configLocation>
+        <suppressionsLocation>checkstyle-config/checkstyle-suppressions.xml</suppressionsLocation>
+        <includeTestSourceDirectory>true</includeTestSourceDirectory>
+        <consoleOutput>true</consoleOutput>
+        <failsOnError>true</failsOnError>
+        <linkXRef>false</linkXRef>
+    </configuration>
+    <executions>
+        <execution>
+            <id>validate</id>
+            <phase>validate</phase>
+            <goals>
+                <goal>check</goal>
+            </goals>
+        </execution>
+    </executions>
+    <dependencies>
+        <dependency>
+            <groupId>com.puppycrawl.tools</groupId>
+            <artifactId>checkstyle</artifactId>
+            <version>10.18.1</version>
+        </dependency>
+    </dependencies>
+</plugin>
+```
+
+For use with github-actions, you will also have to tell `actions/checkout` to also recurse submodules:
+```yaml
+- uses: actions/checkout@v4
+  with:
+    submodules: 'true'
+```
+
+**Configure your IDE:**  
+Go to `Settings` then search for "checkstyle".  
+Add a new checkstyle configuration and select the `checkstyle-config/checkstyle.xml` file.  
+Set the property `org.checkstyle.sun.suppressionfilter.config` to `checkstyle-config/checkstyle-suppressions.xml`
+
 ## Information
 
 About Bits is a company based in South Tyrol, Italy. You can find more information about us on [our website](https://aboutbits.it).