Add README to prepare for release

README.md

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+# changelog-check-action
+
+This GitHub Action will check whether your PR also modifies the changelog. This
+is useful to ensure that each change committed comes with an user-readable
+explanation.
+
+## How it works
+
+1. Create a PR with some changes
+2. See the action fail your build because the `CHANGES.md` wasn't modified
+3. Add an entry in `CHANGES.md` and push
+4. See the action succeed.
+
+Not all changes need a changelog entry, some are not user-visible, like
+changing CI or some internal refactoring. For those it is possible to disable
+the check. Just create a new label, `no changelog` and apply it to the PR, then
+the changelog check will be disabled for this PR.
+
+## Set up
+
+First, set up the action in `.github/workflows/<your-file>.yml`.
+
+### Example
+
+Add this to your GitHub workflow:
+
+```yaml
+- name: Check changelog
+  uses: tarides/changelog-check-action@v1
+```
+
+### Full example
+
+A complete `.github/workflows/changelog.yml` might look like this:
+
+```yaml
+name: Check Changelog
+on:
+  pull_request:
+    types: [assigned, opened, synchronize, reopened, labeled, unlabeled]
+    branches:
+      - main
+jobs:
+  Check-Changelog:
+    name: Check Changelog Action
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: tarides/changelog-check-action@v1
+```