remove text replaced by specification
README.md: SIM must not perform IMSI detach
README.md: improve headlines
README: add user-configurable minimum duration
README.md: fix typo
README.md: use headlines
docs: update to feedback from Neels
Most importantly, there is no ACK SMS anymore, because the next LU isthe implicit ACK. While at it, I've filled in the first TS we probablyneed to update.
Related: OS#440
README/docs: add FIXME: needs to be updated
README.md: describe current approach
Based on my "Approach C" in OS#4400.