update readme to include blurb on decentralization

it occurred to me that someone discovering monero for the first time, through the code and actually reading the readme, would not have any indication of the design focuses of keeping monero decentralized, both in terms of the mining infrastructure and the low cost of node operation.  I know the topic is sort of a ... wishy washy thing, but I tried to craft some language that gets the point across in general terms.

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