Now all modules are in core modules folder

modules/diff/readme.txt

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+
+Diff 2.x for Drupal 6.x
+-----------------------
+Diff enhances usage of node revisions by adding the following features:
+
+- diff between node revisions on the 'Revisions' tab to view all the changes
+  between any two revisions of a node
+- highlight changes inline while viewing a node to quickly see color-coded
+  additions, changes, and deletions
+- preview changes as a diff before updating a node
+
+
+Installation
+------------
+Diff can be installed like any other Drupal module -- place it in
+the modules directory for your site and enable it on the `admin/build/modules`
+page.
+
+Diff needs to be configured to be used with specific node types on your site.
+Enable any of diff's options on a content type's settings page (e.g.
+`admin/content/node-type/page`).
+
+
+Technical
+---------
+- Diff compares the raw data, not the filtered output, making it easier to see
+changes to HTML entities, etc.
+- The diff engine itself is a GPL'ed php diff engine from phpwiki.
+
+API
+---
+This module offers `hook_diff()` which modules may use to inject their changes
+into the presentation of the diff. For example, this is used by
+`content.diff.inc` (see CCK), `upload.inc`, and `taxonomy.inc`.
+
+Maintainers
+-----------
+- dww (Derek Wright)
+- moshe (Moshe Weitzman)
+- rötzi (Julian)
+- yhahn (Young Hahn)