"Receive BTC/USD rate" example added.

Author: dzarezenko

examples/public/btc_usd.php

@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * Receive BTC/USD rate.
+ */
+
+require_once("../../vendor/autoload.php");
+
+use poloniex\api\Poloniex;
+
+// Returns ticker based BTC rate value in USD.
+$tickerRate = Poloniex::getTickerBTCRate();
+
+// Returns estimated (by order book) BTC rate value in USD.
+$estimatedRate = Poloniex::getEstimatedBTCRate();