add explanation for pandas SQL comparison

Author: afeld

readings/week_07.md

@@ -4,5 +4,6 @@
 
 1. [Learn SQL](https://www.codecademy.com/learn/learn-sql)
 1. [pandas: Comparison with SQL](https://pandas.pydata.org/docs/getting_started/comparison/comparison_with_sql.html)
+   - It's written for people coming to pandas _from_ SQL, so you'll need to flip it around mentally.
 
 Now [submit a response](../README.md#responses).