Usage example for custom source

[abhemanyus]

The unzip.Open.s3 function does not support s3 clients like minio, forcing the user to fall back on the custom source implementation.
There is an example for google cloud storage, but none for Minio.

A simple example can be added to the docs:

  const customSource = {
    stream: function(offset: number, length: number) {
      const pass = new PassThrough();
      minioClient.getPartialObject(
        S3_BUCKET_NAME,
        key,
        offset,
        length
      )
        .then(stream => stream.pipe(pass));
      return pass;
    },
    size: async function() {
      const objMetadata = await minioClient.statObject(S3_BUCKET_NAME, key);
      return objMetadata.size;
    }
  };
  const directory = await unzipper.Open.custom(customSource);