add exercise series

I added a bonus to this.  The typically terse and vague readme technically describes two different kinds of results.  It shows a list of all series, and then it says 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, starting with Go's `, ok` idiom, so that is the bonus.  I used the same `-tags` mechanism as with robot-name to let people code and test an improved version.