Update the "Usage" section of the README

Author: bbatsov

README.md

@@ -54,16 +54,22 @@ result will be nothing short of havoc.**
 
 `M-x inf-clojure` or `C-c C-z` within a Clojure source file.
 
-`inf-clojure-program` is a custom variable that defines a program to run
-or the tcp socket to connect to when starting `inf-clojure`. By default
-this variable is set to `lein repl`, which will start a REPL via Leiningen.
-However, it is possible to use a cons pair like `("localhost" . 5555)` to
-connect to a socket REPL like the one provided with [planck](http://planck-repl.org/),
-which can be started from the command line with `planck -n 5555`.
+`inf-clojure` has several custom variables which control the command
+used to start a REPL for particular project type - `inf-clojure-lein-cmd`,
+`inf-clojure-boot-cmd` and `inf-clojure-generic-cmd`.
+
+By default all those variables are set to strings (e.g. `lein repl`).
+However, it is possible to use a cons pair like `("localhost" . 5555)`
+to connect to a socket REPL like the one provided
+with [planck](http://planck-repl.org/), which can be started from the
+command line with `planck -n 5555`.
 
 Use `C-u C-c C-z` to start a REPL with a different command/cons pair than
 the default specified in `inf-clojure-program`.
 
+You can set custom values to `inf-clojure` variables on a per-project basis using [directory
+variables](https://www.gnu.org/software/emacs/manual/html_node/emacs/Directory-Variables.html).
+
 ## ElDoc
 
 `eldoc-mode` is supported in Clojure source buffers and `*inferior-clojure*`