Improve documentation.

README.md

@@ -10,3 +10,27 @@ and it will delete the specified folder and all of its contents. In order to
 walk around limitations in the Windows API, we rename the folders to `@` on
 the way down the tree in order to shorten the path names, so that we don't
 get errors because the path exceeds 250.
+
+## What's the matter?
+
+When working with `npm` it is not unusal to get `node_modules` directory
+trees which are very deep, with paths reaching lengths of 400 or more
+characters.
+
+Trying to delete these folders with the _Windows Explorer_  results in
+error messages complaining about the fact that the _path is too long_ and
+deletion becomes a nightmare.
+
+To circumvent this problem, I've written this tiny naive tool which just
+walks the folder structure, renames every folder to a one-character length
+name while descending the tree, and then deletes everything while moving
+back up the tree.
+
+## I'm left with a folder called @
+
+Sometimes, folders or files are in use (usually by the `explorer` process,
+but this might also be the antivirus kicking in) and the tool cannot delete
+or rename some folders or files.
+
+Usually, the resulting folder structure has already been shortened enough so
+that you can simply delete the remaining garbage with the _Windows Explorer_