INSTALL/README: Clean up the description of how to run the emacs interface.

INSTALL/README: Clean up the description of how to run the emacs interface.

The INSTALL file still had old information about the "make
install-emacs" command which no longer exists. README was also giving
pointers on how to develop a real interface, (which is not the right
thing since README should be addressed to users, not coders).

So remove the stale and misplaced information, and instead add a new
"Running notmuch" section to the README describing how to run the
notmuch command-line interface and how to run the emacs interface.