emacs: instruct user to autoload notmuch instead of require'ing it

When (require 'notmuch) is added to ~/.emacs notmuch is loaded to every
instance of emacs although it may not be used in majority of
those instances.

When (autoload 'notmuch "notmuch" ...) is added to ~/.emacs notmuch
is loaded (only) when user invokes the notmuch function.

User may want to add other entrypoints to notmuch by adding more
autoloads -- the autoload instruction given should offer them clue how
to do so.