README: Mention USB copying of Hex files

[chrysn]

Inspired by https://github.com/nrf-rs/nrf-hal/issues/60 (but not resolving it fully, probably), this adds text to the general examples README on alternatives to the cargo embed process described in the individual READMEs.

This might need a few more items on the list based on the methods listed in #60; I'll be asking there to state which methods are still relevant. (To me, only copying is relevant, as that's what works without going through the process of unlocking the AP lock that is engaged on the nrf42dk board).

[jonas-schievink]

Thanks, this seems like a good addition!

bors r+

[bors[bot]]

Build succeeded:

• ci