Documenting the timestamp for BIP-0030

Per the linked commit, this rule became active for blocks with timestamps after on March 15, 2012 at 00:00 UTC (February 20, 2012 at 00:00 UTC for testnet).

bip-0030.mediawiki

@@ -17,7 +17,7 @@ So far, the Bitcoin reference implementation always assumed duplicate transactio
 To counter this problem, the following network rule is introduced:
 *Blocks are not allowed to contain a transaction whose identifier matches that of an earlier, not-fully-spent transaction in the same chain.
 
-This rule is to be applied to all blocks whose timestamp is after a point in time that is yet to be decided.
+This rule is to be applied to all blocks whose timestamp is after March 15, 2012, 00:00 UTC (testnet: February 20, 2012 00:00 UTC).
 
 ==Rationale==
 Whatever solution is used, the following law must be obeyed to guarantee sane behaviour: the set of usable