added a link to contributing.md#how-to-submit-an-issue to the wiki/RE…

…ADME.md homepage

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minor edits to bitso ccxt#1985

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refix mercado fetchTrades to support for since and to ccxt#1990

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okex fetchMarkets 1ST/USD fix ccxt#2008

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minor edits to the Manual

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removed an error from the Manual confusing the taker and the maker

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1.10.1171

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refix hitbtc2 average is also dependent on last/open prices ccxt#2002

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btctradeua: fetchTicker: close -> last

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addded a minor clarification on the security of API credentials

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