CH12: mention BIP320-style header extension

ch10.asciidoc

@@ -1778,10 +1778,15 @@ in the merkle tree, which means that any change in the coinbase script
 causes the merkle root to change. Eight bytes of extra nonce, plus the 4
 bytes of "standard" nonce allow miners to explore a total 2^96^ (8
 followed by 28 zeros) possibilities _per second_ without having to
-modify the timestamp. If, in the future, miners could run through all
-these possibilities, they could then modify the timestamp. There is also
-more space in the coinbase script for future expansion of the extra
-nonce space.
+modify the timestamp.
+
+Another solution widely used today is to use up to 16 bits of the block
+header versionbits field for mining, as described in BIP320.  If each
+piece of mining equimpent has its own coinbase transaction, this allows
+an individual piece of mining equipment to perform up to 281 TH/s by
+only making changes to the block header.  This keeps mining equipment
+and protocols simpler than incrementing the extranonce in the coinbase
+transaction every 4 billion hashes.
 
 [[mining_pools]]
 ==== Mining Pools