Add some docs to samples/net/ieee802154/hw

[galak]

We don't really have and docs for the sample that tell what it does or doesn't do, how to use it, etc. Would be nice to have a bit of explanation for what going on.

[tbursztyka]

It's quite simple actually: just boots up, setup the 15.4 chip (depending on Kconfig options selected) and runs net and ieee15_4 shell modules. It's really meant for 15.4 q&d testing. I'll make some .rst file.

[galak]

I guess docs on the shell modules would be good, and than something in the sample to ties everything together.

[tbursztyka]

This sample does not exist any-more. closing