Replace image prefix var with input

README.md

@@ -3,4 +3,23 @@
 Read more about this feature here: https://docs.gitlab.com/ee/user/application_security/sast/
 
 Configure SAST with CI/CD variables (https://docs.gitlab.com/ee/ci/variables/index.html).
-List of available variables: https://docs.gitlab.com/ee/user/application_security/sast/index.html#available-cicd-variables
+List of available variables: https://docs.gitlab.com/ee/user/application_security/sast/index.html#available-cicd-variables
+
+## Usage
+
+You should add this component to an existing `.gitlab-ci.yml` file by using the `include:`
+keyword.
+
+```yaml
+include:
+  - component: gitlab.com/gitlab-components/sastg@<VERSION>
+```
+
+where `<VERSION>` is the latest released tag or `main`.
+
+### Inputs
+
+| Input | Default value | Description |
+| ----- | ------------- | ----------- |
+| `stage` | `test`      | The stage where you want the job to be added |
+| `image_prefix` | `$CI_TEMPLATE_REGISTRY_HOST/security-products` | Define where all Docker image are pulled from |