docs: add a description of the solution to lc problem: No.0121 (doocs#748)

No.0121. Best Time to Buy and Sell Stock

Author: YangFong

solution/0100-0199/0121.Best Time to Buy and Sell Stock/README.md

@@ -44,6 +44,8 @@
 
 <!-- 这里可写通用的实现逻辑 -->
 
+参考本站 [面试题 63. 股票的最大利润](/lcof/%E9%9D%A2%E8%AF%95%E9%A2%9863.%20%E8%82%A1%E7%A5%A8%E7%9A%84%E6%9C%80%E5%A4%A7%E5%88%A9%E6%B6%A6/README.md?id=%E8%A7%A3%E6%B3%95)（题意相同）。
+
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 ### **Python3**