docs: complete README with explanation about action usage

README.md

@@ -1,2 +1,51 @@
 # auto-changie
-github action to run changie and generate changelog with automatic numbering
+
+Github action to run [changie](https://changie.dev/) and generate changelog with automatic numbering
+
+## How it works
+
+These are the step done by this composite action :
+
+- Setup node with `inputs.node-version`
+- Setup changie cli through npm with `inputs.changie-version`
+- Initialize changie subtree if not already exist (base on `.changie.yaml` existance)
+- Retrieve all commit since last commit contained in last changie release version's file, or since beginning if not release have been done yet.
+- Create a changie entry of type Changed for each commit of previous step
+- Batch all change in a new automatic version
+- Merge all version and update global `CHANGELOG.md` file
+
+
+## Usage example
+
+```yaml
+name: Changelog generator
+
+on:
+  push:
+    branches:
+      - main
+      - master
+
+jobs:
+  build:
+    if: "contains(github.event.head_commit.message, 'CI') != true"
+    steps:
+      - uses: action/checkout@<latest_tag>
+        with:
+          fetch-depth: 0
+          fetch-tags: false
+          
+      # ... do some CI habitual stuff
+          
+      - uses: RouxAntoine/auto-changie@<latest_tag>
+
+      - name: Push changelog
+        uses: stefanzweifel/git-auto-commit-action@<latest_tag>
+        with:
+          commit_message: "CI: Push updated changelog"
+```
+
+This pipeline clone code update changelog with auto-changie and push it to main thanks to [git-auto-commit-action](https://github.com/stefanzweifel/git-auto-commit-action).
+
+> Notice the use of `fetch-depth: 0` should be set. 
+> Indeed without this not all commit are retrieve and auto-changie fail to find all commit updated since previous run.