add exercise series

I added a bonus to this.  The typically terse and vague readme technically described two different kinds of results.  It showed a list of all series, and then it said if you ask for **_a_** 6-digit..., implying a single result.  I played with the technicality.  In the first case, the correct answer for an impossible query is an empty list.  I handled the second case where "you deserve whatever you get" by not testing the result of an impossible query, allowing freedom to implement "whatever."

Of course we want people to learn out-of-band signaling, so that is the bonus.  I used the same `-tags` mechanism as with robot-name to let people code and test an improved version.