Update README

README.md

@@ -38,3 +38,9 @@ If you want to use this in your own infrastructure, setup your AWS credentials w
 ```
 
 Be aware of S3 transfer costs if your runners are not in the same AWS region as your bucket.
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+## Special environment variables
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+* `RUNS_ON_S3_BUCKET_CACHE`: if set, the action will use this bucket to store the cache.
+* `RUNS_ON_S3_FORCE_PATH_STYLE` or `AWS_S3_FORCE_PATH_STYLE`: if one of those environment variables equals the string `"true"`, then the S3 client will be configured to force the path style.
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