add optimizing parameters examples in docs

docs/source/intro/optimizing_parameters.rst

@@ -4,6 +4,36 @@
 参数调优
 ==================
 
+对于以下双均线策略，我们希望对其进行参数调优，我们可以通过命令行参数 :code:`--extra-vars` 或者通过配置 :code:`extra.context_vars` 传递变量到 :code:`context` 对象中。
+
+.. code-block:: python
+
+    from rqalpha.api import *
+    import talib
+
+
+    def init(context):
+	context.s1 = "000001.XSHE"
+
+	context.SHORTPERIOD = 20
+	context.LONGPERIOD = 120
+
+
+    def handle_bar(context, bar_dict):
+	prices = history_bars(context.s1, context.LONGPERIOD+1, '1d', 'close')
+
+	short_avg = talib.SMA(prices, context.SHORTPERIOD)
+	long_avg = talib.SMA(prices, context.LONGPERIOD)
+
+	cur_position = context.portfolio.positions[context.s1].quantity
+	shares = context.portfolio.cash / bar_dict[context.s1].close
+
+	if short_avg[-1] - long_avg[-1] < 0 and short_avg[-2] - long_avg[-2] > 0 and cur_position > 0:
+	    order_target_value(context.s1, 0)
+
+	if short_avg[-1] - long_avg[-1] > 0 and short_avg[-2] - long_avg[-2] < 0:
+	    order_shares(context.s1, shares)
+
 
 通过函数调用传递参数
 ====================================