Create spec for Proxy.sol

[mds1]

nit: it would be more ideal to have a spec for Proxy.sol and point to that, not sure if we do

Additionally, link to it from the OP Stack Manager specs page

Originally posted by @tynes in #236 (comment)

[tynes]

The following functionality is consensus critical:

    modifier proxyCallIfNotAdmin() {
        if (msg.sender == _getAdmin() || msg.sender == address(0)) {
            _;
        } else {
            // This WILL halt the call frame on completion.
            _doProxyCall();
        }
    }

The || address(0) check in particular, where address(0) counts as having admin access. Hardfork system txs spoof address(0) to modify the predeploys