`defmt` documentation

[SergioGasquez]

Since the usage of espflash with defmt is not straight forward we should document it on the readme(s).

[SergioGasquez]

I gathered some notes on defmt, espflash, esp-println... https://hackmd.io/@SergioGasquez/BJvGEo3Sa The idea is to convert them into official documentation, although I don't know what will be the best format. As usual, any input (both on the notes and how to “publish” them) is more than welcome!