Update installation instructions to use :plugins.

README.md

@@ -12,9 +12,13 @@ using [Leiningen](http://github.com/technomancy/leiningen):
 * Install `clojure-mode` either from
   [Marmalade](http://marmalade-repo.org) or from
   [git](http://github.com/technomancy/clojure-mode).
-* `lein plugin install swank-clojure 1.4.0`
+* Add `[lein-swank "1.4.3"]` to the `:plugins` section of either
+  `project.clj` or your user profile.
 * From an Emacs buffer inside a project, invoke `M-x clojure-jack-in`
 
+If you are still using Leiningen 1.6.2 or earlier, you will need to do
+`lein plugin install swank-clojure 1.4.0` instead.
+
 That's all it takes; there are no extra install steps beyond
 `clojure-mode` on the Emacs side and the `swank-clojure` plugin on the
 Leiningen side. In particular be sure you don't have any other