Discuss the usage of the `log` crate

[tgross35]

It would likely be good to point out in this book that log works in no_std environments, and can have handlers for RTT, semihosting, ITM, UART, etc.

Imho this is kind of important and isn't mentioned anywhere. It's much nicer for supporting crates to just use the log macro and allow the parent binary crate to determine how they should be printed.

[eldruin]

Sounds like a good idea. Would you make a proposal PR?

[tgross35]

Sure, I'll try to do that sometime in the near future.