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 documentation now reflects reality.
Merge pull request #31 from airbreather/patch-1

Documenting the timestamp for BIP-0030