docs: update readme

Author: 2color

README.md

@@ -83,7 +83,7 @@ The signing key and proof will be used as [inputs](#inputs) to the action.
 | `upload-car-artifact`     | Upload and publish the CAR file on GitHub Action Summary pages                                                                                      | `'true'`                                   |
 | `cluster-retry-attempts`  | Number of retry attempts for IPFS Cluster uploads                                                                                                   | `'5'`                                      |
 | `cluster-timeout-minutes` | Timeout in minutes for each IPFS Cluster upload attempt                                                                                             | `'2'`                                      |
-| `cluster-pin-expire-in`   | Time duration after which the pin will expire in IPFS Cluster (e.g. 720h for 30 days)                                                               | -                                          |
+| `cluster-pin-expire-in`   | Time duration after which the pin will expire in IPFS Cluster (e.g. 720h for 30 days). If unset, the CID will be pinned with no expiry.             | -                                          |
 
 ## Outputs