docs: update the dev/diff readme

dev/diff/README.md

@@ -1 +1,22 @@
-# diff
+# Datatracker Diff Tool
+
+This tool facilitates testing 2 different datatracker instances (with their own database) and look for changes using the diff tool. Everything runs in docker containers.
+
+The source instance will use the code from where it is run. The target instance can be a remote tag / branch / commmit or another local folder.
+
+## Requirements
+
+- Node `16.x` or later
+- Docker
+
+## Usage
+
+1. From the `dev/diff` directory, run the command:
+```sh
+npm install
+```
+2. Then run the command:
+```sh
+node cli
+```
+3. Follow the on-screen instructions.