bip-325: document signet minimum difficulty

This was implicit in the genesis block's nbits value, but better to be clearer.

Author: ajtowns

bip-0325.mediawiki

@@ -75,6 +75,15 @@ A block is considered fully validated only if the to_sign transaction is a valid
 
 There is one other acceptable special case: if a block's challenge is e.g. `OP_TRUE` (`0x51`), where an empty solution would result in success, the block is also considered valid if the signet commitment is absent.
 
+=== Minimum Difficulty ===
+
+The minimum difficulty for blocks in signet is approximately
+0.0011265, corresponding to a maximum target of
+<code>00000377ae000000000000000000000000000000000000000000000000000000</code>
+which is represented by an nBits value of <code>0x1e0377ae</code>.
+
+This value is chosen so that multiplying the maximum target by maximum retarget timespan (8 weeks measured in seconds) does not overflow a uint256, making it easier to do difficulty adjustment calculations.
+
 == Genesis Block and Message Start ==
 
 The genesis block is the same for all signet networks, whereas the message start is defined as the first four bytes of the sha256d of the challenge script as a single data push (see below).