Add a README.
Replace the symlinks to headers with the actual files.

Accidentally left the symlinks before committing the initial version.
Need to find a way to synchronise those files with their (future)
optiboot repository versions though.
All a Tx to be started without exiting Rx mode.

The library will remember if Rx was on, automatically switch to idle
before going into Tx, and then restore the Rx mode if needed.  Also
expand the comment about why we're flushing the Tx FIFO everytime.
Let the caller choose Standby or Power Off when going idle.