Update README with parquet dataset documentation

Remove original tar.zst (replaced by 270 parquet shards)

Rename shards 241-270 to 00270 total

Rename shards 211-240 to 00270 total

Rename shards 181-210 to 00270 total

Rename shards 151-180 to 00270 total

Rename shards 121-150 to 00270 total

Rename shards 91-120 to 00270 total

Rename shards 61-90 to 00270 total

Rename shards 31-60 to 00270 total

Rename shards 1-30 to 00270 total

Add parquet shard 270 (298 posts)

Add parquet shard 269 (500,167 posts)

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Add parquet shard 232 (500,002 posts)