docs: clarify usage for tests with jvm system properties

[gaeljw]

Aims to fix #49 by providing users some guidance in the documentation about the expected usage.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

Since Maven is the most popular build system, I think it would be useful to have a copy-paste example for Maven that the user could insert into their project with minimal effort:

<project>
  […]
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <configuration>
          <systemPropertyVariables>
            <mail.smtp.class>org.jvnet.mock_javamail.MockTransport</mail.smtp.class>
            <mail.pop3.class>org.jvnet.mock_javamail.MockStore</mail.pop3.class>
            <mail.imap.class>org.jvnet.mock_javamail.MockStore</mail.imap.class>
          </systemPropertyVariables>
        </configuration>
      </plugin>
    </plugins>
  </build>
</project>

[basil]

Thanks for the PR!