Update README: drush enable installs the plugin in sites/all/libraries

Problem/Motivation

The documentation says that you need to download the Colorbox plugin into the sites/all/libraries folder: https://git.drupalcode.org/project/colorbox/-/tree/7.x-2.x

But this happens automatically when you enable the module, which is great: https://git.drupalcode.org/project/colorbox/-/blob/7.x-2.x/drush/colorbo...

So after these commands, the module is enabled AND the Colorbox plugin placed in the libraries folder:

$ drush dl libraries colorbox
$ drush en libraries
$ drush en colorbox
Colorbox plugin has been installed in sites/all/libraries            [success]
The following extensions will be enabled: colorbox
Do you really want to continue? (y/n): y
colorbox was enabled successfully.                                   [ok]
Thanks for installing Colorbox                                       [status]

Steps to reproduce

Enable the module with Drush, check the sites/all/libraries folder and see that the Colorbox plugin has been downloaded.

Proposed resolution

Update README to reflect this.

Remaining tasks

User interface changes

API changes

Data model changes