Add a README

Author: axel-op

README.md

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+# Agnostic Serverless Functions in Java: Package Action
+
+An [agnostic serverless function](https://github.com/axel-op/agnostic-serverless-functions-java) is a serverless function whose configuration is not linked to a particular FaaS framework.
+
+This action performs the packaging steps needed before deploying a Java function to a specific FaaS provider.
+
+## Usage
+
+```yml
+name: Example workflow
+
+on: push
+
+jobs:
+  example-job:
+    
+    env:
+      FAAS_PROVIDER: gcloud
+      MAVEN_SERVER_ID: github-packages # this is the id set in pom.xml
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up JDK and Maven settings
+        uses: actions/setup-java@v1
+        with:
+          java-version: 11
+          server-id: ${{ env.MAVEN_SERVER_ID }}
+          server-username: MAVEN_USERNAME
+          server-password: MAVEN_PASSWORD
+      - name: Package
+        env:
+          MAVEN_USERNAME: ${{ secrets.MAVEN_USERNAME }}
+          MAVEN_PASSWORD: ${{ secrets.MAVEN_PASSWORD }}
+        uses: axel-op/agnostic-serverless-functions-java-package-action@main
+        with:
+          function-name: name-of-your-function
+          faas-provider: ${{ env.FAAS_PROVIDER }}
+        # working-directory: . # required if different from $GITHUB_WORKSPACE
+
+```
+
+### Inputs
+
+- `function-name`: the name of the function that will be deployed
+- `faas-provider`: the provider the function will be packaged for. Accepted values are: `azure`, `gcloud` (more providers to come).
+
+### About Maven settings
+
+Building an agnostic function [requires Maven artifacts](https://github.com/axel-op/agnostic-serverless-functions-java#configuration) hosted on GitHub Packages. [This README](https://github.com/axel-op/maven-packages) explains how to add them in the dependencies of your Maven project. To retrieve them during the workflow, [the Maven settings file](https://maven.apache.org/settings.html) must contain your credentials to GitHub Packages.
+
+The example above uses [`actions/setup-java`](https://github.com/actions/setup-java) to tell Maven to use the environment variables `MAVEN_USERNAME` and `MAVEN_PASSWORD` as credentials. These environment variables are only set during the packaging step.
+
+## Full example
+
+Check out [this workflow](https://github.com/axel-op/agnostic-serverless-functions-java-example/blob/main/.github/workflows/deployment.yml) for a complete example of a deployment workflow using this action.