Update to EIP-150: Add information about previous gas costs

This is a proposed update to EIP-150 by @chfast.

“Because Yellow Paper only describes the latest mainnet hard fork, there is not official source of information were the gas costs before the changes of EIP-150 are provided.”

The pull request: https://github.com/ethereum/EIPs/pull/1140/commits/2ef0a3f09ffce3ba9b7cfec2d2f20f0001ec6011#diff-9906fa711a4da77e30a336f7692f8ce8

The discussion: https://github.com/ethereum/EIPs/pull/1140