Auto merge of #1499 - arcalinea:document-coinbase-maturity-security, r=bitcartel
Auto merge of #1499 - arcalinea:document-coinbase-maturity-security, r=bitcartel

Note that Coinbase maturity interval does not protect JoinSplits

Changed wording of Block Chain Reorganization section in security-warnings.md to note that we did not change Coinbase maturity #1387, but that this also does not protect JoinSplits from becoming invalidated in the case of a block chain reorg #953

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