Add install instructions as just rbenv plugin.

README.md

@@ -14,6 +14,13 @@ write permission to `/usr/local`, you will need to run `sudo
 ./install.sh` instead. You can install to a different prefix by
 setting the `PREFIX` environment variable.
 
+If you only intend to use ruby-build via rbenv then you can
+install it locally as a plugin:
+
+    $ mkdir -p ~/.rbenv/plugins
+    $ cd ~/.rbenv/plugins
+    $ git clone git://github.com/sstephenson/ruby-build.git
+
 ### Installing Ruby
 
 To install a Ruby version, run the `ruby-build` command with the path