[CM] s7-playground (interactive tutorial) update

[bil at ccrma.Stanford.EDU]

> apologies to Bil for the shameless copy-paste text

no problem -- let me know of any mistakes -- it's hard
to keep documentation up-to-date.

On the github stuff -- I read somewhere (hacker news probably)
that this is a "generational problem" -- old-timers don't
see any benefit in "project management tools" whatever that
means, and young folks can't imagine life without them.
I don't even have a smartphone.