Update README to mention autoloads generation

This is only needed for the "manual installation" of snapshots. Any future proper release tar-balls created by `make dist` will  contain a generated `haskell-site-file.el` file.

This should fix #152 (which was caused by ae7cee0557).
Improve directory prompting and resolution (refs #133, #134, refs #144).
Don't prompt for save if there's no existing session.
Functions to kill and cleanly restart a session's process (refs #61).