README: complete/simplify instructions for Emacs

Insert complete block to copy-paste into .emacs without modifications (uses "opam config var" to get the path to merlin.el and ocamlmerlin). Loads merlin only when needed. Runs merlin by default on all OCaml files (simplification; the user annoyed by that can clearly see she has only two lines to comment out).
README: emacs: simplify deps

No need for json.el AFAICS.
Merge pull request #106 from lehy/patch-2

README: complete/simplify instructions for Emacs