Automatically update docker hub readme (github action) (#679)

* add new github action : update docker hub readme

.github/workflows/update_docker_hub_doc.yml

@@ -0,0 +1,24 @@
+name: Update Docker Hub README
+
+on: 
+  push:
+    branches:
+      - master
+    paths:
+      - 'docker/README.md'
+
+jobs:
+  update-docker-hub-readme:
+    runs-on: ubuntu-latest
+    steps:
+      - 
+        name: Check out the repo
+        uses: actions/checkout@v2
+      -
+        name: Update docker hub README
+        uses: ms-jpq/sync-dockerhub-readme@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: crowdsecurity/crowdsec
+          readme: "./docker/README.md"