Update pom.xml with main class definition.

pom.xml

@@ -12,6 +12,24 @@
             <url>https://m2.dv8tion.net/releases</url>
         </repository>
     </repositories>
+    <plugins>
+        <plugin>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>exec-maven-plugin</artifactId>
+            <version>3.0.0</version>
+            <executions>
+                <execution>
+                    <phase>test</phase>
+                    <goals>
+                        <goal>java</goal>
+                    </goals>
+                    <configuration>
+                        <mainClass>moe.nekojimi.chords.Main</mainClass>
+                    </configuration>
+                </execution>
+            </executions>
+        </plugin>
+    </plugins>
     <dependencies>
         <dependency>
             <groupId>net.dv8tion</groupId>