Add README.md

Author: robinjhuang

README.md

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+# node-diff
+
+A Github Action for diffing custom nodes.
+
+## Description
+
+This action checks out your PR branch and your main branch and compares the custom nodes defined in `NODE_CLASS_MAPPINGS` for backwards incompatible changes.
+
+This helps you develop custom nodes that don't break existing workflows.
+
+## Usage
+
+Just add this to your custom node repo: `.github/workflows/validate.yml`
+
+```
+name: Validate backwards compatibility
+
+on:
+  pull_request:
+    branches:
+      - master
+      - main
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: comfy-org/node-diff@main
+```
+
+If you use a branch other than `main` as your base branch, add that here:
+
+```
+steps:
+  - uses: comfy-org/node-diff@main
+  with:
+    base_ref: 'master'
+```