Explain what package.el is

Author: bbatsov

README.md

@@ -153,8 +153,9 @@ CIDER packs plenty of features. Here are some of them (in no particular order):
 
 ## Installation
 
-The canonical way to install CIDER is via `package.el`, but plenty of other
-options exist. (see the [wiki](https://github.com/clojure-emacs/cider/wiki/Installation)).
+The canonical way to install CIDER is via `package.el` (Emacs's built-in package
+manager), but plenty of other options exist. (see the
+[wiki](https://github.com/clojure-emacs/cider/wiki/Installation)).
 
 ### Prerequisites