Merge pull request #6 from softwareforgood/add-README.md

Add README.md

README.md

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+Check Artifact v4 Existence
+====
+
+A composite Github Action inspired by the very awesome [Check artifact exists](https://github.com/xSAVIKx/artifact-exists-action).
+
+Checks if an artifact with a given name, uploaded with actions/upload-artifact@v4, exists. Only works with the actions/upload-artifact v4 and later, not versions v1-v3.
+
+## Inputs
+
+- **name (required)** - Name of the artifact to check
+
+## Outputs
+
+- **exists** - Whether the artifact exists and could be downloaded. `true` if it the artifact was downloaded, `false` it couldn't be downloaded. It could also be `null` if something  in the composite workflow failed and it never couldn't got to the end, if the `download-artifact` call was skipped
+or something else strange happened.
+
+## Example usage
+
+```yaml
+- uses: softwareforgood/check-artifact-v4-existence@v0
+  with:
+    name: "example-artifact"
+```