found the document `README.parallel` to be a little bit incorrect

Hi hackers,
Recently, I'm looking into the implementation of parallel index building. And I noticed that in `README.parallel` we mentioned that `we allow no writes to the database and no DDL` . But if I understand it correctly, index building as one significant DDL is now supporting parallel (though the `_bt_load` is still single processed). So I guess we should update the outdated document.

     The related file is here. https://github.com/postgres/postgres/blob/8c7146790811ac4eee23fab2226db14b636e1ac5/src/backend/access/transam/README.parallel#L85
[https://opengraph.githubassets.com/e00d7ae4b1ecff53f42d16a5f9dabfdada039cabbfc1a703bad3634a5903d3d0/postgres/postgres]<https://github.com/postgres/postgres/blob/8c7146790811ac4eee23fab2226db14b636e1ac5/src/backend/access/transam/README.parallel#L85>
postgres/README.parallel at 8c7146790811ac4eee23fab2226db14b636e1ac5 · postgres/postgres<https://github.com/postgres/postgres/blob/8c7146790811ac4eee23fab2226db14b636e1ac5/src/backend/access/transam/README.parallel#L85>
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Best,
Qinghao