add README describing domain-code tokenized corpus

domain-code tokenized pretrain corpus (3.42B tokens, 26 shards)

replace legacy corpus with domain-code tokenized data

Tokenized shard 4: +134,222,935 tokens

Tokenized shard 3: +134,217,869 tokens

Tokenized shard 2: +134,229,783 tokens

shard 0001: +134,217,869 tokens

shard 0000: +134,229,783 tokens

initial commit