chore(build): set mainClass in main pom.xml

Doesn't do anything useful because Java (see readme).

pom.xml

@@ -26,6 +26,13 @@
                 <configuration>
                     <source>${jdk.version}</source>
                     <target>${jdk.version}</target>
+										<archive>
+											<manifest>
+												<addClasspath>true</addClasspath>
+												<!-- here we specify that we want to use the main method within the App class -->
+												<mainClass>org.insa.graphs.gui.MainWindow</mainClass>
+											</manifest>
+										</archive>
                 </configuration>
             </plugin>
         </plugins>