docs: Add a README (#10)

* docs: Add a README

* docs: add test badge

* docs: update badge

* docs: remove useless badge

README.md

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+[![build-test](https://github.com/owkin/docker-dependency-updater/actions/workflows/test.yml/badge.svg?branch=main&style=flat-square)](https://github.com/owkin/docker-dependency-updater/actions/workflows/test.yml)
+
+## About
+
+GitHub action to update system dependencies installed in a Dockerfile.
+This action currently supports [alpine](https://hub.docker.com/_/alpine) and [debian](https://hub.docker.com/_/debian) images.
+
+## Usage
+
+In the examples below we are also using other actions:
+ - [`peter-evans/create-pull-request`](https://github.com/peter-evans/create-pull-request) action will create a pull request on the repository if the files are changed in the action context.
+ - [`actions/checkout`](https://github.com/actions/checkout/) to checkout the repository content.
+
+### Update dependencies
+
+Using this you can automatically update dependencies of your docker images in a similar way as using dependabot.
+
+```yaml
+name: Dependencies
+on:
+  schedule:
+    - cron: "0 6 * * *"
+  workflow_dispatch:
+
+jobs:
+  updates:
+    timeout-minutes: 10
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: owkin/docker-dependency-updater@v1
+        with:
+          dockerfile: ./docker/Dockerfile
+          dependencies: ./docker/dependencies.json
+          apply: true
+
+      - uses: peter-evans/create-pull-request@v3
+        with:
+          commit-message: "chore(deps): update dockerfile dependencies"
+          branch: chore/update-docker-dependencies
+          title: "chore(deps): update dockerfile dependencies"
+          body: Updated dependencies.json
+          labels: dependencies
+          delete-branch: true
+```
+
+Note the `workflow_dispatch` trigger that will enable you to run this workflow if the daily update missed the new version and you need to update during the day.
+
+## Customizing
+
+### Action inputs
+
+| Name           | Type   | Description |
+| ---            | ---    | ---         |
+| `dockerfile`   | String | Path to the Dockerfile you want to update |
+| `dependencies` | String | Path to the dependencies.json file where you keep your dependencies pinned |
+| `apply`        | Bool   | If true the action will apply chenges directly to the files checked out |
+
+## Recommendations
+
+Before using this Action you will need to update your Dockerfile in order to extract the dependencies you install.
+The `dependencies.json` file should have the following structure:
+```json
+[
+  {
+    "name": "curl",
+    "version": "7.55.0-r2"
+  }
+]
+```
+
+In your Dockerfiles you can install the dependencies from this file by copying it inside the image and running the update taking this file as an input.
+
+```docker
+FROM alpine/latest
+
+COPY ./dependencies.json /tmp/dependencies.json
+
+RUN apk update && apk add --no-cache jq \
+        && jq -r '.[] | "\(.name)=\(.version)"' /tmp/dependencies.json | xargs apk add --no-cache \
+        && rm /tmp/dependencies.json
+```
+
+One of the downsides is that you have the `jq` package installed in an unpinned manner but since it's only a build dependency, it should be acceptable.
+
+