[request] Add an example of Chisel Printf

[ccelio]

The 2.1 First Module introduces the peekPostTester. It also uses a Scala Println() to show test completion.

It does not however show the most useful testing construct of all --- the magical Printf!

I've gotten feedback from users who keep trying to use Println() to debug their Chisel code and became very confused.

Thanks and keep up the good work!

[grebe]

+1 on having printf somewhere. The difference between chisel printf() and println() is important to explain like other Scala vs Chisel constructs. I have a little PR, not sure if I put it in the right place.