New command that provides explanation / examples for karma giving/receiving

[pogross]

Idea

@karmabot explain karma should provide a (private) message that explains the concept of karma and gives some examples.

Requirements

• When an admin uses the command it should be posted in the channel
• When a regular user uses the command it should be message via private chat to the user that used the command
• When a users joins the slack he should get a explanation along with the welcome message (extending here to cover Explanation of Karma concept in welcome message #58)

[pogross]

Also relevant for #71