Update readme about asset packagist.

README.md

@@ -129,3 +129,26 @@ section of composer.json:
 ### How do I switch from packagist.drupal-composer.org to packages.drupal.org?
 
 Follow the instructions in the [documentation on drupal.org](https://www.drupal.org/docs/develop/using-composer/using-packagesdrupalorg).
+
+### How can I add libraries using composer.json?
+
+It is possible to use libraries with composer with thank to the
+asset-packagist repository (https://asset-packagist.org/).
+
+For example, to use colorbox:
+```
+composer require npm-asset/colorbox:"^0.4"
+
+```
+Composer will detect new versions of the library that meet your constraints.
+In the above example it will download anything from the series of the 0.4.*
+series of colorbox
+
+When managing libraries with composer this way, you may not want to add it to
+version control. In that case, add specific directories to the .gitignore file.
+```
+# Specific libraries (which we manage with composer)
+docroot/libraries/colorbox
+```
+
+For more details, see https://asset-packagist.org/site/about