Added the documentation

docs/index.rst

@@ -19,6 +19,7 @@ Contents:
     Generating thumbnails <thumbnails>
     Validating uploads <validation>
     File import behavior <file-import-behavior>
+    Thumbnail generation shell <thumbnail-shell>
 
 
 Indices and tables

docs/thumbnail-shell.rst

@@ -0,0 +1,24 @@
+Thumbnail shell
+---------------
+
+What it does
+~~~~~~~~~~~~
+The shell will look through your database for images and regenerate the thumbnails based on
+your models Upload behaviour configuration. This allows you to change your thumbnail configuration and run the
+shell to update your images without having to re-upload the image.
+
+How it works
+~~~~~~~~~~~~
+The shell takes the model you provide and checks that the Upload plugin is present and configured. Then it will loop
+though all the images checking that the configured upload field is populated in the database and also ensuring that the
+file exists on the file system. Then it will regenerate the thumbnails using the current model configuration.
+
+Running the shell
+~~~~~~~~~~~~~~~~~
+You can run the shell from the command line as you would any cake shell.
+
+.. code:: bash
+
+    Console/cake upload.thumbnail generate
+
+You will then be asked which model you want to process, and the shell will then process your images.