Adding reference to Maven mascot

[hendrikebbers]

Based on a discussion with the creator of the mascot we must add "The Apache Maven project mascot is trademark the Apache Maven project" for images showing it.

Addition: It is named "Shotgun". Currently I would try to avoid that name since it is not really positive.

[hendrikebbers]

“Apache :trademark: Apache Maven :trademark: and the Apache Maven mascot (Shotgun the owl) are trademarks of the Apache Software Foundation”