Add more information on this package's purpose

Props @aleclarson for the suggestion in johnpbloch#20.

Author: johnpbloch

README.md

@@ -8,6 +8,10 @@
 
 A custom Composer plugin to install WordPress core outside of `vendor`.
 
+This installer is meant to support a rather specific WordPress installation setup in which WordPress is installed in a subdirectory ([see the WordPress codex on that subject](https://codex.wordpress.org/Giving_WordPress_Its_Own_Directory)) and in which the location of `WP_CONTENT_DIR` and `WP_CONTENT_URL` have been customized to be outside of WordPress core ([see the WordPress codex on that subject](https://codex.wordpress.org/Editing_wp-config.php#Moving_wp-content_folder)). This is because composer will delete your whole wp-content directory every time it updates core if you don't separate the two. If that installation setup isn't what you are looking for, then this installer is probably not something you will want to use.
+
+For more information on this site setup and using Composer to manage a whole WordPress site, [check out @Rarst's informational website](https://composer.rarst.net/) which also includes [a site stack example using this package](https://composer.rarst.net/recipe/site-stack/).
+
 ### Usage
 To set up a custom WordPress build package to use this as a custom installer, add the following to your package's composer file: