docs: implement how to usage (#3)

docs/usage.md

@@ -0,0 +1,37 @@
+To use this action, make a file `.github/workflows/confluence.yml`. Here's a template to get started:
+
+```yaml
+name: confluence-sync
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  confluence:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Check out a copy of the repo
+        if: ${{ !env.ACT }}
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Get changed files
+        id: changed-files
+        uses: tj-actions/changed-files@v17.1
+        with:
+          files: |
+            *.md
+          files_ignore: |
+            *.tpl.md
+
+      - name: Sync confluence
+        uses: hadenlabs/action-confluence-sync@0.0.0
+        with:
+          confluence_base_url: '${{ secrets.CONFLUENCE_BASE_URL }}'
+          confluence_user: '${{ secrets.CONFLUENCE_USER }}'
+          confluence_token: '${{ secrets.CONFLUENCE_ACCESS_TOKEN }}'
+          files: '${{ steps.changed-files.outputs.all_changed_files }}'
+```