update README, add docker auth help

Author: Miguel Savignano

README.md

@@ -12,6 +12,37 @@ Execute docker build and push using docker caches.
     build_params: --build-arg=NPM_TOKEN=${{secrets.NPM_TOKEN}}
 ```
 
+Use your own docker authentication for private repositories
+Examples:
+
+Azure
+
+```yml
+- run: az acr login --name myregistry
+- uses: devmasx/cached-docker-build-push@v0.1.0
+  with:
+    image_name: devmasx/project-name
+```
+
+Google cloud
+
+```yml
+- run: gcloud auth configure-docker
+- uses: devmasx/cached-docker-build-push@v0.1.0
+  with:
+    image_name: devmasx/project-name
+```
+
+Docker Hub
+
+```yml
+- run: |
+  docker login -u ${{secrets.DOCKER_USERNAME}} -p ${{secrets.DOCKER_PASSWORD}}
+- uses: devmasx/cached-docker-build-push@v0.1.0
+  with:
+    image_name: devmasx/project-name
+```
+
 ## Docker build
 
 Use the latest docker image for use as cache